fix(sessions): preserve original Event.id from raw_event in VertexAiSessionService (v1)

[chelsealong]

Ports #6531 to the v1 line, as requested by @ashubham there ("The fix works, can we please port to 1.x as well").

Problem

_from_api_event overwrote the ADK Event.id with the last segment of the Vertex event resource name, so the same logical event reported a different id depending on how it was observed — one value while streaming, another after a reload. Anything keyed on Event.id across a reload boundary saw them as two distinct events.

Fix

Stop clobbering id in the event_dict.update({...}) call and fall back to the resource name only when raw_event carries no id:

event_dict.setdefault('id', api_event_obj.name.split('/')[-1])

Older data without a raw_event id keeps the previous behaviour.

Evidence

Identical source change to #6531 (verified with a diff of both patches). Added the same regression test, test_get_session_preserves_original_event_id_from_raw_event, which puts a distinct id in raw_event and asserts get_session returns it rather than the resource name.

$ pytest tests/unittests/sessions/test_vertex_ai_session_service.py::test_get_session_preserves_original_event_id_from_raw_event
1 passed

# with the source change reverted (git stash), to show the test actually proves the fix:
1 failed

$ pytest tests/unittests/sessions/test_vertex_ai_session_service.py
33 passed

Branch is cut from current upstream/v1.

---

Prepared with the help of Claude Code (Anthropic's AI coding assistant); I reviewed the diff and the test output before pushing.