feat: Add contains_dtype() method for Schema

[Kevin-Patyk]

This is an old one, but it resolves #10247.

• It adds contains_dtype() to SchemaEx in polars-core/src/schema/mod.rs and a helper function DataType::contains_dtype() in polars-core/src/datatypes/dtype.rs.
• It adds an identical method to to the class Schema(BaseSchema) on the Python side.
• Also, included a new file, test_schema.py to test the functionality on the Python side.

The Python Schema class is a pure Python class (not a PyO3-bound #[pyclass]), so contains_dtype() was implemented directly in Python, reusing the existing unpack_dtypes() utility for the recursive case.

🤖 Claude Opus 4.6 was used to help me navigate the codebase.

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.39%. Comparing base (74e50a9) to head (fa812f1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-core/src/datatypes/dtype.rs	0.00%	10 Missing ⚠️
crates/polars-core/src/schema/mod.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26661      +/-   ##
==========================================
- Coverage   81.40%   81.39%   -0.02%     
==========================================
  Files        1794     1794              
  Lines      245190   245210      +20     
  Branches     3080     3081       +1     
==========================================
- Hits       199609   199591      -18     
- Misses      44795    44833      +38     
  Partials      786      786              

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