fix(python): pin an available pylance wheel on macOS

## Problem

The Python test extra pins `pylance==15.0.0b7` for every platform. On macOS, pip cannot resolve that pin: the configured `lance-format` index publishes b7 wheels for Linux and Windows but not macOS, while public PyPI returns HTTP 404 for the version and the `lancedb` index contains no 15.x release. The macOS Python dependency installation consequently cannot resolve the pin.

## Change

- Keep the exact `pylance==15.0.0b7` test pin on non-macOS platforms.
- Use `pylance==15.0.0b4` on macOS. This is the newest release in this range with a macOS wheel (`cp310-abi3-macosx_11_0_arm64`).
- Refresh `python/uv.lock` for the platform-specific resolutions.

The production Rust/Lance inputs are unchanged: `Cargo.lock` is byte-identical to `main` and remains on Lance `15.0.0-beta.7`. No tests, assertions, coverage, or CI checks are removed or weakened.

## Validation

- Confirmed the wheel identity from the configured index and matched its published SHA-256 to `uv.lock`; wheel metadata reports `Name: pylance`, `Version: 15.0.0b4`.
- Verified the b4 APIs used by the Python tests (`blob_field`, `blob_array`, `BlobType`, `Blob`, `dataset`, `write_dataset`, `FFILanceTableProvider`, and `DirectoryNamespace`). The Python test suite does not reference the b7-only `mark_sstables_as_compacted`, `mem_wal_dir_id`, `mem_wal_index_details`, `clustering_id`, `LanceFragment`, `IndexSegment`, `UpdateConfig`, or `max_source_fragments` changes.
- Verified scalar read/write interoperability in both directions between b4 and the beta.7 LanceDB native binding, plus beta.7 reads of a b4-written blob dataset with byte and null fidelity.
- `uv lock --check` with both configured Fury indexes and `unsafe-best-match`.
- CI dependency tests.
- Ruff 0.9.9 format and lint checks.
- Targeted Python tests: 911 passed, 22 skipped.
- Full Python suite (`-m "not slow and not s3_test"`): 2858 passed, 157 skipped, 70 deselected.

python/pyproject.toml

@@ -64,7 +64,9 @@ tests = [
     "polars>=0.19, <=1.32.3",
     "pyarrow<25",
     "pyarrow-stubs>=16.0",
-    "pylance==15.0.0b7",
+    # pylance 15.0.0b5+ has not published macOS wheels.
+    "pylance==15.0.0b7; sys_platform != 'darwin'",
+    "pylance==15.0.0b4; sys_platform == 'darwin'",
     "requests>=2.31.0",
     "datafusion>=54,<55",
     "opentelemetry-sdk>=1.30.0",