fix(sessions): preserve original Event.id from raw_event in VertexAiSessionService

[chelsealong]

Link to Issue or Description of Change

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

• Closes: VertexAiSessionService.get_session replaces Event.id with the Vertex resource name, discarding the persisted raw_event.id #6530

Problem:
VertexAiSessionService.get_session returns each Event.id as the Vertex
event resource name (api_event_obj.name.split('/')[-1]) instead of the
original ADK Event.id that was generated during the run. The original id
is persisted inside raw_event, but _from_api_event deep-copies
raw_event and then unconditionally overwrites id before validating. As a
result the same logical event has two different ids depending on whether it
is observed live (streaming) or reloaded via get_session, which breaks any
caller that correlates events by id across the two paths.

Solution:
In _from_api_event, when raw_event is present, keep the id already in
raw_event and only fall back to the Vertex resource name if raw_event
doesn't have one (older data written before this field existed).

Testing Plan

Unit Tests:

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

Added test_get_session_preserves_original_event_id_from_raw_event in
tests/unittests/sessions/test_vertex_ai_session_service.py, which asserts
that get_session returns the id stored in raw_event rather than the
Vertex resource name suffix. Verified the new test fails against the
pre-fix code (AssertionError: ... assert '658731057016733696' == 'c452feb0-b9ad-48b1-b863-c7dd2f085378') and passes with the fix applied.

$ pytest tests/unittests/sessions/test_vertex_ai_session_service.py -v
...
45 passed, 5 warnings in 4.09s

pre-commit run --files src/google/adk/sessions/vertex_ai_session_service.py tests/unittests/sessions/test_vertex_ai_session_service.py also passes.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective.
• New and existing unit tests pass locally with my changes.

Additional context

This change was prepared with AI coding assistance (Claude Code) and reviewed by me before submission.