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/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) {