fix: address ds-review-bot v6/v7 findings on the image-input assembly

- resolveLlmRoute: reuse the yml pi-ai row for providers it already routes
  (DUPLICATE_ADAPTER boot failure) and detect an unset model by origin, not
  by comparison against one deployment default; covered by a new spec.
- LlmService.resolveModelInfoFor preserves (and validates) modality
  metadata, arming the host image preflight for exact-route resolution.
- session.selectModel refuses a text-only target once the session log
  carries an image on any replayed route; an accepted switch would strand
  every later turn with no in-product recovery.
- The composer no longer gates image intake on the handshake activeModel
  snapshot (wrong authority for a per-session decision); the host preflight
  plus the error strip own capability, deployment limits stay client-side.
- InputHub shell teardown releases the scope's draft images (File objects
  and object URLs leaked for the page lifetime).
- session.prompt image parts carry optional alt into the durable block;
  ImageBlock documents assistant-side rendering as forward compatibility.
- Assembled built-client lane apps/web/tests/image-display.snapshot.ts pins
  the history galleries over the authorized attachment route, the lightbox,
  and the composer paste rail; the attachment rail is an accessible group.
- Docs: validateImage on the seam page, fixture byte metadata matches its
  PNG, and the Agent Note claims now match the shipped coverage.

packages/client/ui-conversation/tests/service-orchestration.spec.ts

@@ -20,13 +20,12 @@ async function bench() {
   })
   // config.input is required (the apply shares its hub with the inject
   // factories); the bench passes its own instance explicitly.
-  const fiber = runtime.ctx.plugin(ConversationService, {
-    input: new InputHub(runtime.ctx),
-  })
+  const hub = new InputHub(runtime.ctx)
+  const fiber = runtime.ctx.plugin(ConversationService, { input: hub })
   await fiber.await()
   const root = runtime.ctx.get('conversation') as ConversationService
   const scoped = runtime.sessions.scope('s1')!.get('conversation') as ConversationService
-  return { runtime, root, scoped, prompt, cancel, loadOlder }
+  return { runtime, hub, root, scoped, prompt, cancel, loadOlder }
 }
 
 describe('ConversationService', () => {
@@ -50,6 +49,26 @@ describe('ConversationService', () => {
     await b.runtime.dispose()
   })
 
+  it('releases draft images when the session scope is disposed', async () => {
+    const b = await bench()
+    const created = vi.spyOn(URL, 'createObjectURL').mockReturnValue('blob:draft-1')
+    const revoked = vi.spyOn(URL, 'revokeObjectURL').mockReturnValue(undefined)
+    try {
+      const [attachment] = b.root.createDraftImages([new File([new Uint8Array(4)], 'a.png', { type: 'image/png' })])
+      if (attachment === undefined) throw new Error('draft attachment missing')
+      // Land the id in the session shell exactly as the composer does.
+      b.hub.shell(b.runtime.sessions.behavior('s1').sessionId).addImages([attachment.id])
+      await b.runtime.sessions.remove('s1')
+      // Scope teardown released the service-held File and its object URL.
+      expect(b.root.draftImages([attachment.id])).toEqual([])
+      expect(revoked).toHaveBeenCalledWith('blob:draft-1')
+    } finally {
+      created.mockRestore()
+      revoked.mockRestore()
+    }
+    await b.runtime.dispose()
+  })
+
   it('fails loudly from the root scope, on an unbound session, or without SessionsService', async () => {
     const b = await bench()
     await expect(b.root.send('x', 'queue')).rejects.toThrow(/requires a session scope/)