feat(python): Add collect_schema method to LazyFrame and DataFrame

[stinodego]

Ref #16328

Changes

• Add collect_schema method to LazyFrame and DataFrame. This method is the same as the .schema property now. For LazyFrame, this is the idiomatic way to resolve the schema.
• Update references to the LazyFrame properties .schema, .dtypes, .columns, and .width to use .collect_schema() instead.

This includes some performance improvements for methods that were abusing LazyFrame properties internally (LazyFrame.describe/fill_null). That convinces me this is a good move.

I did not yet add any warnings to the LazyFrame properties. This will be done separately.

[codecov]

Codecov Report

Attention: Patch coverage is 86.84211% with 5 lines in your changes missing coverage. Please review.

Project coverage is 81.07%. Comparing base (8965a68) to head (5eb079d).
Report is 3 commits behind head on main.

Files	Patch %	Lines
py-polars/polars/lazyframe/frame.py	80.00%	1 Missing and 3 partials ⚠️
py-polars/polars/io/parquet/functions.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16929      +/-   ##
==========================================
- Coverage   81.10%   81.07%   -0.03%     
==========================================
  Files        1435     1435              
  Lines      189546   189553       +7     
  Branches     2712     2711       -1     
==========================================
- Hits       153732   153686      -46     
- Misses      35314    35367      +53     
  Partials      500      500              

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