Add PolarsExpr type annotation for pydantic validation

[NeejWeej]

without subclassing.

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

on the pydantic documentation.

[github-actions]

Test Results

436 tests  +10   432 ✅ +10   23s ⏱️ -1s
  1 suites ± 0     4 💤 ± 0 
  1 files   ± 0     0 ❌ ± 0 

Results for commit 95c02a3. ± Comparison against base commit fa733a0.

This pull request removes 6 and adds 16 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[PolarsExpression]
ccflow.tests.exttypes.test_polars ‑ test_expression_complex[typ1]
ccflow.tests.exttypes.test_polars ‑ test_expression_from_string[PolarsExpression]
ccflow.tests.exttypes.test_polars ‑ test_expression_from_string[typ1]
ccflow.tests.exttypes.test_polars ‑ test_expression_passthrough[PolarsExpression]
ccflow.tests.exttypes.test_polars ‑ test_expression_passthrough[typ1]
ccflow.tests.exttypes.test_polars ‑ test_json_serialization_roundtrip[PolarsExpression]
ccflow.tests.exttypes.test_polars ‑ test_json_serialization_roundtrip[typ1]
ccflow.tests.exttypes.test_polars ‑ test_model_field_and_dataframe_filter
ccflow.tests.exttypes.test_polars ‑ test_polars_expression_validate_errors
…

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

❌ Patch coverage is 97.22222% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.45%. Comparing base (fa733a0) to head (95c02a3).

Files with missing lines	Patch %	Lines
ccflow/tests/exttypes/test_polars.py	97.01%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
+ Coverage   95.42%   95.45%   +0.02%     
==========================================
  Files         120      120              
  Lines        6754     6792      +38     
  Branches      427      427              
==========================================
+ Hits         6445     6483      +38     
  Misses        203      203              
  Partials      106      106              

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[ptomecek]

I think it would be better to make PolarsExpression the annotated type and just drop PolarsExpr. Having both is confusing.