fix(vector-stores): pass custom Weaviate filters

[haosenwang1018]

What changed

• Stop hardcoding Weaviate filters to only user_id, agent_id, and run_id.
• Reuse one filter builder for search(), keyword_search(), and list().
• Preserve valid falsy metadata filter values like 0 while still skipping None.
• Add a regression test that custom metadata keys are included in the Weaviate filter.

Fixes #4053.

Note: #4848 was closed in favor of #4853, but #4853 fixed same-key metadata filter merging in Memory; it does not change the Weaviate vector store whitelist that still drops custom metadata keys.

Testing

• uv run --with pytest --with pytest-mock --with ruff --with weaviate-client --with python-dotenv python -m pytest tests/vector_stores/test_weaviate.py -q
• uv run --with ruff ruff check mem0/vector_stores/weaviate.py tests/vector_stores/test_weaviate.py
• uv run --with ruff ruff format --check mem0/vector_stores/weaviate.py tests/vector_stores/test_weaviate.py