feat(python): optimize union factory and visit methods to avoid .dict() serialization

[tjb9dc]

Description

Refs customer feedback about performance bottlenecks in discriminated union factory and visit methods.

Link to Devin run: https://app.devin.ai/sessions/ac18896281bc4c41a46e21d637ce328f
Requested by: [email protected] (@tjb9dc)

Changes Made

This PR optimizes the factory and visit methods for discriminated unions to avoid expensive .dict() serialization/deserialization. Previously, these methods would:

1. Call .dict(exclude_unset=True) on the source model (deep serialization)
2. Spread the dict into a new model constructor (re-validation)

The new implementation uses shallow field copying via copy_and_update_model, which:

• Directly copies field values from source.__dict__
• Preserves fields_set / __pydantic_fields_set__ for exclude_unset semantics
• Handles extra fields (fields not in schema)
• Uses construct/model_construct to bypass validation (matching original behavior)

Files changed:

• Added copy_and_update_model utility to all 4 pydantic utility variants
• Updated discriminated_union_with_utils_generator.py to emit optimized code
• Added export and getter in core_utilities.py
• Updated seed test outputs for unions fixture

Updates since last revision

• Fixed mypy no-any-return errors by wrapping return values with typing.cast(Model, ...) in all 4 pydantic utility variants

Human Review Checklist

• Verify copy_and_update_model correctly handles fields_set tracking (lines 288-323 in pydantic_utilities.py)
• Verify Pydantic v1 vs v2 code paths are correct (the v1_on_v2 variants use pydantic.v1.BaseModel)
• Check the generator changes in discriminated_union_with_utils_generator.py - especially the make_visitor_argument_for_same_properties function and write_copy_and_update closure
• Review generated code in seed tests (e.g., union_with_base_properties.py) to confirm the optimization is applied correctly
• Note: The diff includes unrelated _build_url test additions in test_http_client.py - these appear to be from a merge/rebase and may need to be reviewed separately

Testing

• Seed tests pass for unions fixture (all 3 configurations: no-custom-config, union-utils, union-naming-v1)
• Lint checks pass (pnpm run check)
• Mypy type checking passes
• May want to run additional seed tests for other fixtures with discriminated unions

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