Optimize FindQuery.update() with partial writes

[lh0156]

Summary

Implements #777 by updating matching records without materializing full Pydantic models.

What changed

• Uses FT.SEARCH ... NOCONTENT to collect matching keys page by page.
• Validates and serializes update values once, including Pydantic constraints and Redis OM storage conversions.
• Uses partial HSET mappings for HashModel.
• Uses path-level JSON.SET commands for JsonModel, including nested __ paths.
• Preserves the use_transaction flag and returns the number of updated records.
• Keeps query projections out of the key-only search and preserves KNN state when copying queries.
• Documents the new return value.

Testing

• pytest -q tests --ignore tests/test_benchmarks.py: 265 passed.
• pytest -q tests/test_hash_model.py tests/test_json_model.py: 147 passed.
• Generated sync tests for the new coverage plus Hash/JSON model tests: 157 passed.
• ruff check and ruff format --check on changed source/tests.
• python -m compileall -q aredis_om tests.
• bandit -q -r aredis_om/model/model.py -s B608.

Redis Stack was run locally via Docker Compose for the integration tests; benchmark tests were excluded from the full run.