Update PolarsExpression to just be a type annotation on pl.Expr

[NeejWeej]

without subclassing.

This follows the approach highlighted here: https://docs.pydantic.dev/latest/concepts/types/#handling-third-party-types

on the pydantic documentation.

This will be a breaking change because PolarsExpression no longer has a validate classmethod

[github-actions]

Test Results

443 tests  +2   439 ✅ +2   25s ⏱️ -1s
  1 suites ±0     4 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit c8aaf03. ± Comparison against base commit 5013e5e.

This pull request removes 6 and adds 8 tests. Note that renamed tests count towards both.

ccflow.tests.exttypes.test_polars.TestPolarsExpression ‑ test_expression
ccflow.tests.exttypes.test_polars.TestPolarsExpression ‑ test_expression_complex
ccflow.tests.exttypes.test_polars.TestPolarsExpression ‑ test_expression_deserialization
ccflow.tests.exttypes.test_polars.TestPolarsExpression ‑ test_json_serialization
ccflow.tests.exttypes.test_polars.TestPolarsExpression ‑ test_validation_eval_failure
ccflow.tests.exttypes.test_polars.TestPolarsExpression ‑ test_validation_failure

ccflow.tests.exttypes.test_polars ‑ test_expression_complex
ccflow.tests.exttypes.test_polars ‑ test_expression_from_string
ccflow.tests.exttypes.test_polars ‑ test_expression_passthrough
ccflow.tests.exttypes.test_polars ‑ test_json_serialization_roundtrip
ccflow.tests.exttypes.test_polars ‑ test_model_field_and_dataframe_filter
ccflow.tests.exttypes.test_polars ‑ test_model_field_and_dataframe_with_columns
ccflow.tests.exttypes.test_polars ‑ test_validation_eval_failure
ccflow.tests.exttypes.test_polars ‑ test_validation_failure

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

❌ Patch coverage is 96.49123% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.13%. Comparing base (5013e5e) to head (c8aaf03).
⚠️ Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
ccflow/tests/exttypes/test_polars.py	96.29%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
+ Coverage   95.12%   95.13%   +0.01%     
==========================================
  Files         126      126              
  Lines        7158     7178      +20     
  Branches      470      470              
==========================================
+ Hits         6809     6829      +20     
  Misses        231      231              
  Partials      118      118              

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