fix(llm): size unknown models and refuse a section that cannot be served

Three defects surfaced while driving the Models page.

A hand-declared model needed an explicit contextWindow and maxTokens,
but a provider listing usually returns ids and nothing else — so the
page happily wrote a profile the adapter then rejected, which took the
whole namespace down silently. Capacities now fall back to the route's
`defaultContextWindow` (262,144) and `defaultMaxTokens` (32,768). Both
are guesses by construction, which is why they are route fields a
deployment corrects once rather than constants buried in the adapter;
the fallback sizes the model and never becomes a per-request cap.

That silent failure was the second defect. A schema-valid profile the
adapter could not serve was stored and only rejected later, disabling
every route in the namespace with nothing said. `dsh-settings` gains an
optional `validate` on registration — a check for what a schema cannot
express — and `llm-pi-ai` refuses an unserviceable section at the write
that produced it. A stored section that fails keeps the namespace's last
good value, as a schema failure already did, so an externally edited
document still cannot strand the owner. The plugin's own last-good
fallback goes with it: nothing reaching it can fail any more.

Third, a model with no reasoning metadata advertised the single level
`off`, which pi-ai translates to *omitting* the reasoning option — the
same request naming no effort produces. Selecting it disabled nothing,
so a provider whose default is to think kept thinking with `off` shown
as selected. Such a model now reports no reasoning capability at all,
which is the seam's way of saying the control is unavailable, and the
per-model `reasoning` flag is gone: without a thinkingLevelMap to spell
levels it could only invent them.

The protocol table narrows to the three a hand-declared route reaches
today, most-reached first so a surface offering a choice defaults to the
one gateways actually speak.

packages/llm/llm-pi-ai/tests/dynamic-config.spec.ts

@@ -146,13 +146,16 @@ describe('request-level dynamic profiles', () => {
     expect(ctx.llm.listProviders().map(provider => provider.id)).toEqual(['openai'])
   })
 
-  it('keeps the last good profiles when a settings snapshot names an unknown provider', async () => {
+  it('refuses a settings write this adapter could not serve, leaving its routes alone', async () => {
     const dir = await home()
     const ctx = await boot(dir, { providers: { openai: {} } })
 
-    // Schema-valid but catalog-invalid: the resolver rejects it and the
-    // last good route set keeps serving.
-    await ctx.settings.update(NS, { providers: { 'not-a-real-provider': {} } })
+    // Shape-valid but unserviceable: a route the catalog does not ship and
+    // that lists no models of its own. The section schema resolves the whole
+    // profile set, so this is refused where it is written rather than stored
+    // and then quietly disabling every route in the namespace.
+    await expect(ctx.settings.update(NS, { providers: { 'not-a-real-provider': {} } }))
+      .rejects.toThrow(/resolves no models/)
     expect(ctx.llm.listProviders().map(provider => provider.id)).toEqual(['openai'])
   })