test(ui-models): cover the failure paths, and share the one message reader

The per-file coverage gate caught three uncovered paths in the error handling
this round added: the page banner for a failed row removal, the editor card's
transport-rejection catch, and `store.fail` itself.

Two of them are one click each — Remove with a rejecting write, Apply with a
rejecting write — so they are covered through the UI rather than by calling
the helpers directly. The third was a duplicated `error instanceof Error ?
error.message : String(error)` in two files; it becomes one exported
`messageOf`, which removes the branch from both call sites and gives the
fallback arm a home a direct unit test can reach (the lint rule forbids
rejecting a promise with a non-Error, so a rejection cannot exercise it).

packages/client/ui-models/src/client/ModelsSection.tsx

@@ -12,6 +12,7 @@ import { useState } from 'react'
 import type { ReactNode } from 'react'
 import type { IApiClient, SettingsNamespaceView } from '@deepseek-ai/dsh-client-connection/client'
 import type { SnapshotSelectorHook } from '@deepseek-ai/dsh-client-web-react'
+import { messageOf } from './store.ts'
 import type { ModelsSettingsState, ModelsSettingsStore, ProviderRow } from './store.ts'
 import { ProviderEditor } from './ProviderEditor.tsx'
 import type { en } from './locales.ts'
@@ -68,7 +69,7 @@ export async function removeProviderProfile(
   } catch (error) {
     // The transport rejected rather than answering; the caller must be able
     // to say so instead of the row silently staying put.
-    return error instanceof Error ? error.message : String(error)
+    return messageOf(error)
   }
   if (!response.result.ok) return response.result.error.message
   await controller.load()

packages/client/ui-models/src/client/ProviderEditor.tsx

@@ -18,7 +18,7 @@ import type { CredentialView, IApiClient, SettingsNamespaceView, SettingsPathOpV
 import {
   deletePath, getPath, nodeAtPath, rehydrateSchema, setPath, validateDraft,
 } from '@deepseek-ai/dsh-client-schema-form'
-import { deriveKeyRef } from './store.ts'
+import { deriveKeyRef, messageOf } from './store.ts'
 import type { en } from './locales.ts'
 import styles from './ModelsSection.module.css'
 
@@ -215,7 +215,7 @@ export function ProviderEditor(props: ProviderEditorProps): ReactNode {
       // A transport failure (disconnect, a request the host refuses) rejects
       // rather than answering; without this the card would stay busy forever
       // with no error shown.
-      setFailure(error instanceof Error ? error.message : String(error))
+      setFailure(messageOf(error))
     } finally {
       setBusy(false)
     }

packages/client/ui-models/src/client/store.ts

@@ -40,6 +40,17 @@ export interface ModelsSettingsState {
   namespaces: ReadonlyMap<string, SettingsNamespaceView>
 }
 
+/**
+ * Human text for a rejected wire call. A transport failure rejects with an
+ * Error; a host or a runtime can reject with anything, and the page still has
+ * to say something.
+ * @param error - the rejection value.
+ * @returns the message to show.
+ */
+export function messageOf(error: unknown): string {
+  return error instanceof Error ? error.message : String(error)
+}
+
 /**
  * Derive the conventional credential reference for a provider route: the v1
  * page never asks for an environment-variable name, so a typed key stores