Merge #5273

### Link to Issue or Description of Change

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

- Related: #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:**

- [x] 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

- [x] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [x] I have performed a self-review of my own code.
- [ ] I have commented my code, particularly in hard-to-understand areas.
- [x] 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.
- [x] 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.

COPYBARA_INTEGRATE_REVIEW=#5273 from petrmarinec:fix-vertex-session-filter-escaping 4d14a1d
PiperOrigin-RevId: 903926570