Add pre-commit hooks with pyink and mypy

[jayendra-info]

This commit sets up automated code quality checks using pre-commit:

1. Pre-commit configuration (.pre-commit-config.yaml):

   • Added pyink for code formatting (reads config from pyproject.toml)
   • Added mypy for type checking
   • Added standard pre-commit hooks (trailing whitespace, end-of-file, etc.)

2. Updated pyproject.toml:

   • Added mypy and pre-commit to dev dependencies
   • Added pandas-stubs for better type checking
   • Configured mypy with per-module ignores for libraries without stubs
   • Set python_version to 3.11 to match .python-version file

3. Fixed type errors in xarray_sql/df.py:

   • Fixed tuple concatenation type issues in block_slices()
   • Fixed Callable type annotation syntax in from_map_batched()
   • Added type ignore for xarray return type in pivot()

4. Updated CONTRIBUTING.md:

   • Documented pre-commit setup steps
   • Updated formatting/type-checking instructions

5. Updated GitHub Actions lint workflow:

   • Added mypy type checking step
   • Changed to use uv run for consistency

6. Auto-fixed formatting issues:

   • Removed trailing whitespace
   • Fixed end-of-file newlines
   • Applied pyink formatting

All hooks pass successfully and type checking works correctly.