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/host/apiproxy/src/api/sessions.schema.ts

@@ -198,7 +198,7 @@ export const imageMediaTypeSchema = z.union([
 /** Prompt wire content is intentionally narrower than merge-extensible durable core content. */
 export const promptContentPartSchema = z.discriminatedUnion('type', [
   z.object({ type: z.literal('text'), text: z.string() }),
-  z.object({ type: z.literal('image'), mediaType: imageMediaTypeSchema, data: z.string(), name: z.string().optional() }),
+  z.object({ type: z.literal('image'), mediaType: imageMediaTypeSchema, data: z.string(), name: z.string().optional(), alt: z.string().optional() }),
 ])
 
 /** session.prompt request payload. */