fix(memory): keep thought parts out of Memory Bank events - #7851

Link to Issue or Description of Change

• Fixes VertexAiMemoryBankService sends the model's thought parts to Memory Bank #7850

Problem:

VertexAiMemoryBankService passes event.content to ingest_events and
memories.generate unchanged, so the model's thought parts
(Part.thought=True) reach Memory Bank as conversation content. Memories can
then be extracted from the model's private reasoning. An event whose only text
is a thought is sent too, because _is_empty_content counts any part.text.

# A model event with parts:
#   [Part(text="the user sounds anxious, keep the reply short", thought=True),
#    Part(text="Here is the answer.")]
# and a second event whose only part is that thought.
await memory_service.add_session_to_memory(session)

# Before: 901 [(<thought>, True), ('Here is the answer.', None)]
#         902 [(<thought>, True)]
# After:  901 [('Here is the answer.', None)]

InMemoryMemoryService, VertexAiRagMemoryService (#7473) and
SqliteMemoryService (#7691) already exclude thoughts.

Solution:

Add _without_thoughts, which returns a copy of the content without thought
parts, and use it in both direct-contents paths. _should_filter_out_event
checks the filtered content, so an event with nothing left is skipped. The
session's own events are not modified, and content without thoughts is passed
through as before.

add_memory is unchanged: it takes MemoryEntry content from the caller
rather than session events.

Testing Plan

Unit Tests:

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

Added to test_vertex_ai_memory_bank_service.py, covering both paths:

• Ingest path (add_session_to_memory): thought parts are dropped, a
  thought-only event is skipped, and the session's events are unchanged.
• Ingest path (add_events_to_memory): a thought-only event alone sends no
  events.
• Generate path: thought parts are dropped and the caller's events are
  unchanged.
• Generate path: no call is made when every event is thought-only.

All 4 new tests fail without the source change and pass with it. Changing the
event content in place instead of copying it fails the "unchanged" checks on
both paths.

$ pytest tests/unittests/memory/test_vertex_ai_memory_bank_service.py
62 passed

$ pytest tests/unittests/memory tests/unittests/tools/test_load_memory_tool.py tests/unittests/tools/test_preload_memory_tool.py
224 passed

pre-commit run passes on the changed files, and mypy reports no issues in
vertex_ai_memory_bank_service.py.

Manual End-to-End (E2E) Tests:

I don't have a Memory Bank instance, so I ran the reproduction script from
#7850 against the mocked agentplatform.Client. On main both events are sent
with their thought parts; with this change only Here is the answer. is sent.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• 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. (Mocked client only; see above.)
• Any dependent changes have been merged and published in downstream modules (N/A; no dependencies).