fix(retrieval): release agent session before episode wait

[suguanYang]

Summary

Closes #399. Prevent agentic retrieval from reusing a request-scoped SQLAlchemy session after a long external agent/LLM episode.

• Roll back the route session before run_episode.
• Use one fresh session for finish-reference resolution, hydration, assembly, and trace persistence.
• Add contract coverage for the long-wait session boundary and isolation parameters.
• No database timeout change or generic SQL retry.

Verification

• uv run pytest apps/api/tests/contract/test_retrieval_contract.py -q — 15 passed
• uv run pytest packages/shared-python/shared/tests/test_agent_explore_harness.py -q — 17 passed
• Ruff — passed
• Pyright — passed
• Full repository suite is currently blocked during collection by pre-existing worker import-path errors.