fix: release retrieval sessions before workflow waits

[suguanYang]

Summary

• Stop the agentic retrieval route from holding the request DB session across workflow planner/navigation waits.
• Move workflow planner budget inventory and final reference hydration into fresh short DB sessions.
• Add RETRIEVAL_WORKFLOW_PLANNER_TIMEOUT_SECONDS with a default of 25.0, and restrict planner fallback to timeout/provider/budget/invalid-plan failures.
• No public API schema, database migration, worker, or storage contract changes.

Verification

• uv run pytest apps/api/tests/contract/test_retrieval_workflow_session_contract.py -q
• uv run pytest apps/api/tests/contract/test_retrieval_contract.py -q
• uv run ruff check packages/shared-python/shared/services/retrieval/execution/routes.py packages/shared-python/shared/services/retrieval/workflow/orchestrator.py packages/shared-python/shared/services/retrieval/workflow/plan_service.py packages/shared-python/shared/services/retrieval/workflow/planner.py packages/shared-python/shared/services/retrieval/workflow/runtime_config.py packages/shared-python/shared/core/config/ai.py apps/api/tests/contract/test_retrieval_workflow_session_contract.py apps/api/tests/contract/test_retrieval_contract.py
• uv run pyright packages/shared-python/shared/services/retrieval/execution/routes.py packages/shared-python/shared/services/retrieval/workflow/orchestrator.py packages/shared-python/shared/services/retrieval/workflow/plan_service.py packages/shared-python/shared/services/retrieval/workflow/planner.py packages/shared-python/shared/services/retrieval/workflow/runtime_config.py packages/shared-python/shared/core/config/ai.py apps/api/tests/contract/test_retrieval_workflow_session_contract.py apps/api/tests/contract/test_retrieval_contract.py
• git diff main...HEAD --check
• Not run: full repository test suite.

Deployment Notes

• New optional environment variable: RETRIEVAL_WORKFLOW_PLANNER_TIMEOUT_SECONDS, default 25.0.
• No database migrations.
• No queue, storage, SDK, or OpenAPI compatibility changes.
• Rollback is code-only: revert the PR to restore previous session lifetime and planner fallback behavior.

Checklist

• Tests were added or updated when behavior changed
• Public docs, examples, or OpenAPI contracts were updated when needed
• Database migrations are idempotent and safe to deploy
• Logs, errors, and validation paths avoid leaking secrets or user data
• The pull request description explains any breaking or user-visible change