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/tests/e2e/run_celery.py

@@ -220,11 +220,11 @@ def _patch_llm_bindings() -> None:
             fake_create_chat_litellm_from_config,
         ),
         (
-            "app.tasks.chat.stream_new_chat.create_chat_litellm_from_agent_config",
+            "app.tasks.chat.streaming.flows.shared.llm_bundle.create_chat_litellm_from_agent_config",
             fake_create_chat_litellm_from_agent_config,
         ),
         (
-            "app.tasks.chat.stream_new_chat.create_chat_litellm_from_config",
+            "app.tasks.chat.streaming.flows.shared.llm_bundle.create_chat_litellm_from_config",
             fake_create_chat_litellm_from_config,
         ),
     ]