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/api/routes/agent_setup.py

@@ -175,7 +175,7 @@ def _build_system_prompt(
         else ""
     )
     template, prompt_obj = get_prompt_or_fallback(
-        "journey_system_v2", _JOURNEY_SYSTEM_PROMPT
+        "journey_system", _JOURNEY_SYSTEM_PROMPT
     )
     compiled = compile_prompt(
         template,