[Data] Fix MCAP datasource import for better compatibility (#57964)

## Description

This PR fixes the import statement in `MCAPDatasource` to follow Python
best practices for explicit imports.

**Change**: Updated from `import mcap` + `mcap.reader.make_reader(f)` to
`from mcap.reader import make_reader` for cleaner, more explicit
importing.

**Why this is needed**: 
- Follows Python best practices for explicit imports (PEP 8)
- Improves code readability by making it clear what's being used
- Better compatibility with different mcap package versions
- Reduces namespace pollution

## Related issues

None - this is a code quality improvement.

## Additional information

### What changed

**Before:**
```python
import mcap

reader = mcap.reader.make_reader(f)
```

**After:**
```python
from mcap.reader import make_reader

reader = make_reader(f)
```

### Impact

- **Minimal risk**: This is a refactoring of the import statement only
- **No API changes**: The `MCAPDatasource` public API remains unchanged
- **No behavioral changes**: The functionality is identical
- **Tested**: All ruff lint checks pass

### File modified

- `python/ray/data/_internal/datasource/mcap_datasource.py` (2 lines
changed)

This change makes the code more maintainable and follows modern Python
conventions for explicit imports.

Signed-off-by: soffer-anyscale <[email protected]>

Author: soffer-anyscale

python/ray/data/_internal/datasource/mcap_datasource.py

@@ -139,9 +139,9 @@ def _read_stream(self, f: "pyarrow.NativeFile", path: str) -> Iterator[Block]:
         Raises:
             ValueError: If the MCAP file cannot be read or has invalid format.
         """
-        import mcap
+        from mcap.reader import make_reader
 
-        reader = mcap.reader.make_reader(f)
+        reader = make_reader(f)
         # Note: MCAP summaries are optional and iter_messages works without them
         # We don't need to validate the summary since it's not required