fix(langfuse): remove invalid user_id/session_id kwargs from start_as_current_observation

Langfuse V3 does not accept user_id/session_id on observation-level calls.
Moved to metadata dict in agent_runner, deep_agent, and agent_setup.

refactor(tests): fixture-based pattern for agent_runner_v2 eval tests

- cases.yaml + data/ fixtures under tests/fixtures/agent_runner_v2/
- pytest_generate_tests parametrizes test_eval_runner from YAML
- _resolve_projects() handles symbolic names and inline dicts
- _evaluate_case() centralizes all assertion logic
- --runner-dir CLI option for custom fixture folders

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/core/agent_runner.py

@@ -251,7 +251,7 @@ async def _run_agent_with_tools(
         lf.start_as_current_observation(
             as_type="span",
             name=agent_name,
-            user_id=user_id or None,
+            metadata={"user_id": user_id} if user_id else None,
             input=user_message,
         )
         if lf else None