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/connection/src/client/fixture.ts

@@ -108,7 +108,9 @@ const FIXTURE_IMAGE_DATA = 'iVBORw0KGgoAAAANSUhEUgAAAKAAAABaCAYAAAA/xl1SAAAAvklE
 const FIXTURE_IMAGE_REF: ImageAttachmentRef = {
   attachmentId: 'fixture:image' as AttachmentIdType,
   mediaType: 'image/png',
-  bytes: 68,
+  // Matches the decoded FIXTURE_IMAGE_DATA exactly (the real backend serves
+  // verified metadata; a mismatched fixture would mislead comparisons).
+  bytes: 247,
   width: 160,
   height: 90,
   name: 'fixture-image.png',