Quote user_id literals in VertexAiSessionService list filters

[petrmarinec]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Related: Quote user_id literals in VertexAiSessionService list filters #5270

2. Or, if no issue exists, describe the change:

Problem:
VertexAiSessionService.list_sessions() built an AIP-160 filter by interpolating raw user_id into a quoted string literal. Embedded quotes could break out of that literal and append additional filter syntax.

Solution:
Quote the user_id value before constructing the filter string so embedded quotes and backslashes stay inside the literal. Added a regression test that captures the exact filter passed to the Vertex client for a quote-containing payload.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Passed locally:

• PYTHONPATH=src pytest tests/unittests/sessions/test_vertex_ai_session_service.py
  • Result: 29 passed

Passed in clean Linux Docker (python:3.11-bookworm):

• pip install -e '.[test]'
• PYTHONPATH=src pytest tests/unittests/sessions
  • Result: 170 passed

Manual Validation:

• On current origin/main, a payload of attacker" OR user_id!="" produced the filter string user_id="attacker" OR user_id!="".
• On this branch, the same payload produces user_id="attacker\" OR user_id!=\"\"", keeping the metacharacters inside the quoted literal.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

This is a small, focused fix that keeps the current filter construction approach but ensures user_id values remain data instead of altering the filter expression.