fix(cli): normalize trigger user ids for sessions

[MukundaKatta]

Summary

• normalize Pub/Sub subscription and Eventarc source metadata before reusing them as session user ids
• replace slash-separated resource paths with path-safe -- delimiters while preserving the full resource identity
• add trigger endpoint regression tests that verify the created sessions are stored under the normalized user ids

Testing

• python3 -m py_compile src/google/adk/cli/trigger_routes.py tests/unittests/cli/test_trigger_routes.py
• python3 -m pytest tests/unittests/cli/test_trigger_routes.py -k "path_safe or with_subscription_metadata or source_from_ce_header" (fails during collection in this environment: ModuleNotFoundError: No module named 'fastapi')

[adk-bot]

Response from ADK Triaging Agent

Hello @MukundaKatta, thank you for creating this PR!

This PR is a bug fix, could you please associate the github issue with this PR? If there is no existing issue, could you please create one?

This information will help reviewers to review your PR more efficiently. Thanks!