feat: add provider-neutral handoff routing hints

[adamweeks]

Summary

• define a provider-neutral normalize_handoff() contract that any connector can use for summary and routing_hint
• use GECX EndSession.metadata as the first adapter; forward only allowlisted summary and routing_hint metadata on one TRANSFER_TO_AGENT event
• document Flow Designer mapping, approved Case-to-queue mapping, and fallback human queue

Verification

• venv/bin/python run_tests.py (494 passed)
• venv/bin/python -m pytest tests/test_handoff.py tests/test_gecx_connector.py tests/test_wxcc_gateway_server.py -q (175 passed)
• venv/bin/ruff check --target-version py38 --select E,F,I src/utils/handoff.py

Deployment

No dev deployment or Flow Designer change was made in this PR.