Linked Issue

Closes #4490

Description

actor_id metadata gets overwritten during memory UPDATE operations when a different actor triggers the update. The preservation condition "actor_id" not in new_metadata in _update_memory() never
triggers because new_metadata always contains the caller's actor_id (passed through from _build_filters_and_metadata() via deepcopy(metadata)).

This breaks actor-level memory isolation in multi-actor scenarios (shared user_id with per-actor actor_id). After a different actor triggers an UPDATE, querying by the original creator's actor_id
returns empty results.

Fix: Remove the "actor_id" not in new_metadata guard so actor_id is always preserved from the existing memory. This treats actor_id as memory ownership rather than last-updater. The history table
already tracks all contributors via db.add_history().

Applied to both sync and async _update_memory() code paths.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor (no functional changes)
• Documentation update

Breaking Changes

N/A

Test Coverage

• I added/updated unit tests
• I added/updated integration tests
• I tested manually (describe below)
• No tests needed (explain why)

Added two tests covering both sync and async paths: test_update_preserves_actor_id_when_different_actor_updates and test_async_update_preserves_actor_id_when_different_actor_updates. All 36 tests
pass locally.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have added tests that prove my fix/feature works
• New and existing tests pass locally
• I have updated documentation if needed