fix(FN-2355): improve AI route diagnostics and stabilize related tests

- Add structured ai-session diagnostics for summarize-title and agent-generation error paths in dashboard routes
- Emit debug-gated summarize request/model resolution diagnostics when FUSION_DEBUG_AI is enabled
- Add route tests that assert diagnostics payloads for summarize and agent generation failures
- Reduce test flakiness by increasing core Vitest timeouts and relaxing brittle extension-discovery argument matching

packages/core/src/migration.test.ts

@@ -125,7 +125,7 @@ describe("FirstRunDetector", () => {
       } finally {
         await testCentral.close();
       }
-    });
+    }, 15_000);
 
     it("should return fresh-install when central DB exists but is unreadable", async () => {
       const tempProjectDir = useIsolatedCwd("kb-corrupt-central-");

packages/core/src/plugin-loader.ts

@@ -88,7 +88,7 @@ export class PluginLoader extends EventEmitter<{
   /** Cache of dynamically imported modules */
   private loadedModules: Map<string, unknown> = new Map();
 
-  /** Monotonic counter for deterministic cache-busting import URLs */
+  /** Monotonic nonce to guarantee unique cache-busting import URLs. */
   private importNonce = 0;
 
   constructor(private options: PluginLoaderOptions) {

packages/core/vitest.config.ts

@@ -22,6 +22,10 @@ export default defineConfig({
     maxWorkers,
     poolOptions: { threads: { minThreads: 1, maxThreads: maxWorkers }, forks: { minForks: 1, maxForks: maxWorkers } },
     fileParallelism: true,
+    // Core runs a large SQLite-heavy suite while other workspace packages test concurrently.
+    // Use a slightly higher timeout to reduce nondeterministic slow-machine flakes.
+    testTimeout: 15_000,
+    hookTimeout: 15_000,
     coverage: {
       enabled: false,
       reporter: ["text", "html", "json"],