refactor(chat): delete legacy stream_new_chat monolith (cutover complete)

The flows orchestrators (new_chat/resume_chat) are now the sole live path
after the byte-for-byte differential proof, so the monolith and its
monolith-vs-flows parity scaffolding are removed.

- Repoint the last live importer (anonymous_chat_routes) to
  streaming.agent.event_loop.stream_agent_events + shared.stream_result.StreamResult
  (drop-in; the keyword-only fallback-commit params default to inert for anon).
- Repoint e2e launcher patch targets to flows.shared.llm_bundle.
- Repoint helper unit tests (chunk_parts, thinking-step ids, tool-input
  streaming) to their flows homes to preserve coverage.
- Delete the monolith, the contract test, and the parity tests
  (parallel_refactor, stage_1, stage_2, orchestrator_frame) whose sole
  purpose was comparing against the now-removed monolith.

Full suite green (2622 passed, 1 skipped); the two excluded live-app dirs
(document_upload, composio) have a pre-existing, env-gated registration 404
unrelated to this change.

surfsense_backend/app/routes/anonymous_chat_routes.py

@@ -356,7 +356,8 @@ async def stream_anonymous_chat(
         from app.db import shielded_async_session
         from app.services.new_streaming_service import VercelStreamingService
         from app.services.token_tracking_service import start_turn
-        from app.tasks.chat.stream_new_chat import StreamResult, _stream_agent_events
+        from app.tasks.chat.streaming.agent.event_loop import stream_agent_events
+        from app.tasks.chat.streaming.shared.stream_result import StreamResult
 
         accumulator = start_turn()
         streaming_service = VercelStreamingService()
@@ -419,7 +420,7 @@ async def stream_anonymous_chat(
 
                 stream_result = StreamResult()
 
-                async for sse in _stream_agent_events(
+                async for sse in stream_agent_events(
                     agent=agent,
                     config=langgraph_config,
                     input_data=input_state,