feat: handle mid-stream LLM resets and deduplicate UI action stream events

- Backend (LLM Gateway):
  - Record structured 'llm_stream_reset' event with discard action, failure reason, and retry messaging on mid-stream generation errors.
  - Add unit test verifying stream reset event payload on mid-stream errors.

- Frontend (Showcase UI):
  - Handle 'llm_stream_reset' events in agent service and stream aggregator, discarding interrupted chunks while showing retry indicators.
  - Establish a single source of truth for tool and action events in the timeline to eliminate duplicated action rows while retaining ADB and explorer tools.
  - Add @angular/cdk dependency and update agent stream component styling and layout.
  - Add unit tests for stream aggregation and tool display filtering.

apps/showcase_ui/package.json

@@ -10,6 +10,7 @@
   },
   "private": true,
   "dependencies": {
+    "@angular/cdk": "^22.1.5",
     "@angular/common": "^22.1.4",
     "@angular/compiler": "^22.1.4",
     "@angular/core": "^22.1.4",