fix(sessions): validate session_id and enforce ownership in delete_session

[nicola-pesavento]

Summary

session_id was interpolated raw into the Vertex AI REST URL, and delete_session ignored its user_id parameter. A frontend-supplied session_id could therefore:

1. Traverse paths via ".." / "..?force=true" / "../../../datasets/<id>" — httpx resolves ../ before the request is sent, so the Vertex AI API receives a clean path pointing to a sibling resource (including the parent reasoning engine itself, cascade-deletable with ?force=true).
2. Delete another user's session (delete-IDOR) — delete_session accepted user_id but never used it. get_session already had an ownership check; delete_session did not.

Reported privately to Google VRP (issue 425431410); reporter was given the green light to disclose publicly.

Changes

• _validate_session_id(): regex ^[A-Za-z0-9_-]+$ applied at every interpolation site (create_session when client supplies an ID, get_session, delete_session, append_event). Matches server-generated session IDs and client-supplied UUIDs; rejects /, .., ?, etc.
• delete_session: fetches the session first and verifies user_id, mirroring the existing check in get_session. Returns silently on 404 to keep delete idempotent.

Test plan

• pytest tests/unittests/sessions/test_vertex_ai_session_service.py — 34 passed (32 existing + 2 new).
• Full unit test suite — 5344 passed, 1 skipped, 0 failed.
• test_delete_session_rejects_other_users_session — cross-user delete blocked, session intact.
• test_session_id_path_traversal_rejected — .., ../foo, ..?force=true, a/b, "" all raise ValueError on get_session and delete_session.

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Hello @nicola-pesavento, thank you for your contribution!

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Thanks!

[rohityan]

Hi @nicola-pesavento , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Please fix formatting errors by running autoformat.sh

[nicola-pesavento]

Hi @nicola-pesavento , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Please fix formatting errors by running autoformat.sh

Hi @rohityan, thanks for the review! Just pushed 2e17f99 with pre-commit run --all-files applied.