test(ai-chat): safety-critical coverage + a11y + pure refactors

Unit tests for the safety-critical paths: crypto secret-box (round-trip,
tamper detection, wrong key), the SSRF guard (blocked ranges + DNS-rebinding),
the ai-chat tools service, the page-embedding repo, and the
assistant-parts/serialization helpers. Those server helpers (assistantParts,
rowToUiMessage, serializeSteps) are exported ONLY for the tests — no runtime
change.

Also: keyboard a11y on the chat history header and conversation rows
(role/tabIndex/Enter+Space), and DRY refactors that move shared logic into one
place (isToolPart -> tool-parts util; buildInitialValues in the MCP form).

The behaviour-changing edits that previously rode along in this commit are
split out into the following two commits, per review.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/server/src/database/repos/ai-chat/page-embedding.repo.spec.ts

@@ -0,0 +1,26 @@
+import { PageEmbeddingRepo } from './page-embedding.repo';
+import type { KyselyDB } from '../../types/kysely.types';
+
+/**
+ * Unit test for the pure access-scoping branch of searchByEmbedding: when the
+ * caller has NO accessible spaces (`spaceIds` empty), the method must early-
+ * return [] WITHOUT touching the database. We inject a db whose query builder
+ * throws if invoked, so any DB access fails the test.
+ *
+ * NOTE: the dimension-mixing case (filter by model_dimensions) needs a live
+ * pgvector-enabled Postgres and is intentionally NOT covered here — it requires
+ * a real DB and is out of scope for this pure unit test.
+ */
+describe('PageEmbeddingRepo.searchByEmbedding', () => {
+  it('early-returns [] for empty spaceIds without any DB call', async () => {
+    const throwingDb = {
+      selectFrom: () => {
+        throw new Error('DB should not be queried for empty spaceIds');
+      },
+    } as unknown as KyselyDB;
+
+    const repo = new PageEmbeddingRepo(throwingDb);
+    const result = await repo.searchByEmbedding('ws-1', [0.1, 0.2, 0.3], [], 10);
+    expect(result).toEqual([]);
+  });
+});