fix(web-config): close the wire boundary, the redacted-replace data loss, and three P2s

Five findings from the #939 review, each reproduced before being fixed.

**Configuration reads are as privileged as writes.** `settings.describe`
returns every exposed namespace's configuration and `credentials.describe`
reports whether an arbitrary environment-variable name is configured and from
where — reconnaissance no anonymous caller should have. Both join
PRIVILEGED_METHODS, so the whole configuration plane is loopback-only until
real authentication exists; `trustedHosts` was never authentication. The model
catalog stays reachable: it carries no endpoints or key state, and a LAN
client's model picker legitimately needs it. Asserted over a real HTTP server,
because the Host header a browser actually sends is what decides this.

**The proxy serves only namespaces a registered model provider addresses.**
The settings seam is general — any plugin may register one — but the Web
configuration plane is the model-provider surface. Without the gate, every
future `settings.register()` would silently become remotely readable and
writable configuration. An unregistered namespace and an unexposed one answer
identically, so no caller can enumerate the registry one probe at a time.

**Path-addressed writes replace the redacted-document rebuild.** The editor
reads the REDACTED descriptor, so rebuilding a section from it and replacing
wholesale deleted every literal secret the wire never returned — reproduced as
`{baseURL, reasoning}` in, stored `apiKey` gone out. `settings.mutate` applies
set/unset ops to the section as it stands at the front of the seam's write
queue, and the client names only fields it can see, so an unseen secret is
untouched by construction rather than by care.

P2s in the same pass: `llm/adapters-updated` now contains async listener
rejections (an uncontained one escaped as unhandledRejection, contradicting
the documented "observer failures are contained"); llm-deepseek's retry-policy
swap uses the atomic `registration.replace` instead of dispose-then-register,
which published `[]` then `["deepseek-official"]` so an observer saw the
provider disappear and come back; and a transport rejection no longer strands
the page in `loading` or a card in `busy`, with removal failures surfaced on
the page banner instead of swallowed.

packages/host/apiproxy/tests/api-proxy-config.spec.ts

@@ -148,6 +148,8 @@ const AdapterConfig = z.object({
 async function harness(options?: {
   settings?: false | { doc?: Record<string, unknown>; readOnly?: boolean }
   credentials?: false | { shadowed?: string[] }
+  /** Skip the directory registration to exercise a namespace the proxy does not expose. */
+  configurableProviders?: false
 }): Promise<Context> {
   const ctx = new Context()
   await ctx.plugin(SessionStore)
@@ -158,6 +160,13 @@ async function harness(options?: {
   await ctx.plugin(LlmService)
   if (options?.settings !== false) await ctx.plugin(MemorySettings, options?.settings)
   if (options?.credentials !== false) await ctx.plugin(MemoryCredentials, options?.credentials)
+  // The proxy serves only namespaces a configurable provider addresses, which
+  // is what the real LLM plugins declare at load; the tests mirror that.
+  if (options?.configurableProviders !== false) {
+    ctx.llm.registerConfigurableProviders([
+      { provider: 'deepseek-official', displayName: 'DeepSeek', settingsNs: 'llm-deepseek', settingsPath: [] },
+    ])
+  }
   // Host-stream opener reads the committed-workspace baseline; the stub
   // suffices — the real workspace composition is api-proxy-workspace.spec's.
   ctx.provide('workspace', { list: () => [] } as never)
@@ -213,6 +222,41 @@ describe('settings domain', () => {
     expect(JSON.stringify(value)).not.toContain('user-secret')
   })
 
+  it('serves only namespaces a registered model provider addresses', async () => {
+    // The settings seam is general: any plugin may register a namespace for
+    // its own configuration. The Web configuration plane is not — it is the
+    // model-provider surface, and a namespace nothing in the provider
+    // directory addresses must be invisible and unwritable here, so a future
+    // plugin cannot become remotely configurable just by registering.
+    const ctx = await harness()
+    ctx.settings.register(NS, AdapterConfig)
+    ctx.settings.register(settingsNamespace('some-other-plugin'), z.object({ secretPath: z.string() }))
+    const api = createApiProxy(ctx, DEFAULTS)
+
+    const value = expectOk(await api.settings.describe(request({})))
+    expect(value.namespaces.map(view => view.ns)).toEqual(['llm-deepseek'])
+
+    for (const response of [
+      await api.settings.update(request({ ns: 'some-other-plugin', patch: { secretPath: '/etc/shadow' } })),
+      await api.settings.replace(request({ ns: 'some-other-plugin', section: {} })),
+    ]) {
+      const error = expectErr(response)
+      expect(error.code).toBe('settings-not-exposed')
+      expect(error.details).toEqual({ ns: 'some-other-plugin' })
+    }
+    // The write never reached the seam.
+    expect(ctx.settings.describe().find(d => String(d.ns) === 'some-other-plugin')?.value).toEqual({})
+  })
+
+  it('refuses even a model-provider namespace once its directory entry is gone', async () => {
+    const ctx = await harness({ configurableProviders: false })
+    ctx.settings.register(NS, AdapterConfig)
+    const api = createApiProxy(ctx, DEFAULTS)
+    expect(expectOk(await api.settings.describe(request({}))).namespaces).toEqual([])
+    expect(expectErr(await api.settings.update(request({ ns: 'llm-deepseek', patch: { baseURL: 'https://x' } }))).code)
+      .toBe('settings-not-exposed')
+  })
+
   it('updates the user layer, answers with the new redacted view, and broadcasts the frame', async () => {
     const ctx = await harness()
     ctx.settings.register(NS, AdapterConfig, { base: { baseURL: 'https://base' } })
@@ -238,7 +282,6 @@ describe('settings domain', () => {
 
   it.each([
     ['an invalid namespace name', 'Not A Namespace', {}],
-    ['an unregistered namespace', 'unknown-ns', {}],
     ['a schema-invalid patch', 'llm-deepseek', { baseURL: 42 }],
   ])('rejects %s as settings-rejected', async (_case, ns, patch) => {
     const ctx = await harness()
@@ -249,6 +292,21 @@ describe('settings domain', () => {
     expect(error.details).toEqual({ ns })
   })
 
+  it('answers an unregistered namespace exactly like an unexposed one', async () => {
+    // Deliberately indistinguishable: separating "does not exist" from
+    // "exists but is not yours to configure" would let a caller enumerate the
+    // registered namespaces one probe at a time.
+    const ctx = await harness()
+    ctx.settings.register(NS, AdapterConfig)
+    ctx.settings.register(settingsNamespace('some-other-plugin'), z.object({ secretPath: z.string() }))
+    const api = createApiProxy(ctx, DEFAULTS)
+    const unknown = expectErr(await api.settings.update(request({ ns: 'unknown-ns', patch: {} })))
+    const unexposed = expectErr(await api.settings.update(request({ ns: 'some-other-plugin', patch: {} })))
+    expect(unknown.code).toBe('settings-not-exposed')
+    expect(unexposed.code).toBe(unknown.code)
+    expect(unexposed.message.replace('some-other-plugin', 'unknown-ns')).toBe(unknown.message)
+  })
+
   it('maps a read-only provider refusal onto the same rejection', async () => {
     const ctx = await harness({ settings: { readOnly: true } })
     ctx.settings.register(NS, AdapterConfig)
@@ -303,7 +361,7 @@ describe('credentials domain', () => {
 
 describe('llm domain', () => {
   it('merges the configurable directory with live routes and appends undeclared ones', async () => {
-    const ctx = await harness()
+    const ctx = await harness({ configurableProviders: false })
     ctx.llm.registerConfigurableProviders([
       { provider: 'deepseek-official', displayName: 'DeepSeek', settingsNs: 'llm-deepseek', settingsPath: [] },
       { provider: 'openai', displayName: 'openai', settingsNs: 'llm-pi-ai', settingsPath: ['providers', 'openai'] },