fix(memory): do not leak temp file when RAG upload fails

[agharsallah]

Problem

VertexAiRagMemoryService.add_session_to_memory creates a
tempfile.NamedTemporaryFile(delete=False) and only removes it via
os.remove on the success path. Two failure paths leak the file
permanently:

1. The ValueError("Rag resources must be set.") raised when no rag
   resources are configured — the temp file is already on disk by then.
2. Any exception from rag.upload_file — the os.remove at the end is
   never reached.

Fix

• Move the rag-resource validation before the temp file is created,
  so nothing is written to disk when the config is invalid.
• Wrap the upload loop and os.remove in try/finally so the temp file
  is always deleted, even when the upload raises.

Tests

Added two cases to test_vertex_ai_rag_memory_service.py:

• test_add_session_removes_temp_file_when_upload_fails — mocks
  rag.upload_file to raise and asserts the temp file is gone afterward.
• test_add_session_does_not_create_temp_file_without_rag_resources —
  asserts no temp file is created when rag resources are unset.

All tests pass.