fix(python): only add pydantic-core dependency for explicit v2 mode

[jsklan]

Description

Fixes a bug where pydantic-core was unconditionally added as a dependency to all generated Python SDKs, breaking compatibility for users who want to use Pydantic v1.

Link to Devin run: https://app.devin.ai/sessions/28614b85508c4423a875ee8848ab1fca
Requested by: @jsklan

Changes Made

• Modified the SDK, Pydantic model, and FastAPI generators to only add pydantic-core as a dependency when the Pydantic version is explicitly set to v2
• For "both" mode, pydantic-core is a transitive dependency of pydantic>=2.0 and doesn't need to be explicitly listed
• For v1 mode, pydantic-core doesn't exist
• For v1_on_v2 mode, we use the pydantic.v1 compatibility layer
• Added version 4.50.1 changelog entry

Updates since last revision

Fixed mypy type-checking errors that occurred because mypy tries to resolve imports at type-checking time even when they're inside runtime conditionals:

• Added # type: ignore[import-not-found] comments to all pydantic_core imports in the core utilities
• Made the top-level PydanticUndefined import in unchecked_base_model.py conditional with a None fallback for non-v2 mode

Testing

• Pre-commit hooks pass
• Seed tests pass in CI

Human Review Checklist

• Verify the conditional check uses the correct variable name in each file (self._version in SDK vs self._pydantic_compatibility in others)
• Confirm the PydanticUndefined = None fallback in unchecked_base_model.py is safe (it's only used inside if IS_PYDANTIC_V2: blocks)
• Verify seed tests pass for Python SDK fixtures with different Pydantic version configurations

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