Scope Vertex RAG memory display names

[petrmarinec]

Summary

Fixes #5294.

This PR tightens VertexAiRagMemoryService memory scoping by replacing the ambiguous dot-delimited RAG display_name format with an encoded v1 format for new uploads. Search results are now parsed into exact app_name, user_id, and session_id components before being accepted.

The change also keeps compatibility for old unambiguous legacy display names in the exact app.user.session form, while ignoring ambiguous legacy names that contain extra dot-delimited components.

Why

The previous client-side filter used:

context.source_display_name.startswith(f"{app_name}.{user_id}.")

That can collide for IDs such as alice and alice.smith, allowing memory stored under demo.alice.smith.* to pass a lookup for demo / alice.

Tests

PYTHONPATH=src python -m pytest tests/unittests/memory -q
42 passed, 2 warnings

python -m isort --check-only src/google/adk/memory/vertex_ai_rag_memory_service.py tests/unittests/memory/test_vertex_ai_rag_memory_service.py
passed

python -m pyink --check --config pyproject.toml src/google/adk/memory/vertex_ai_rag_memory_service.py tests/unittests/memory/test_vertex_ai_rag_memory_service.py
passed

git diff --check -- src/google/adk/memory/vertex_ai_rag_memory_service.py tests/unittests/memory/test_vertex_ai_rag_memory_service.py
passed

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.