fix(python): Fix Python 3.14 compatibility with Pydantic v1 datetime parsing

[jsklan]

Description

Refs square/square-python-sdk#188, cohere-ai/cohere-python#706

SDKs generated by the Python generator emit warnings and may break on Python 3.14 because pydantic.v1.datetime_parse is incompatible with Python 3.14's new type annotation semantics (PEP 649/749). This PR removes the problematic imports and uses Pydantic V2-native alternatives instead.

Link to Devin run: https://app.devin.ai/sessions/037a40e643704fd1bd779214fa20c335
Requested by: @jsklan

Changes Made

Across all 4 pydantic_utilities.py template variants (shared/, with_pydantic_aliases/, with_pydantic_v1_on_v2/, with_pydantic_v1_on_v2/with_aliases/):

• Replaced from pydantic.v1.datetime_parse import parse_date/parse_datetime with pydantic.TypeAdapter-based parse_date()/parse_datetime() functions that use TypeAdapter.validate_python() for date/datetime coercion
• Wrapped all remaining pydantic.v1.* imports (ModelField, ENCODERS_BY_TYPE, get_args, get_origin, is_literal_type, is_union) in warnings.catch_warnings() to suppress the Python 3.14 UserWarning
• Added # type: ignore[attr-defined] annotations to TypeAdapter lines for mypy compatibility when type-checking against pydantic v1 stubs
• Updated versions.yml for python-sdk (4.55.2), pydantic (1.11.3), and fastapi (2.1.1) generators
• Updated all corresponding seed test snapshots (368 files)

The else branch (native pydantic v1 — only used when IS_PYDANTIC_V2 is False) is intentionally left unchanged, as pydantic v1 itself does not support Python 3.14.

Human Review Checklist

• Verify TypeAdapter.validate_python() is functionally equivalent to the old parse_date/parse_datetime for all input types (strings, datetime objects, etc.)
• Important: In with_pydantic_v1_on_v2 variants, imports changed from pydantic.X to pydantic.v1.X (e.g., pydantic.fields.ModelField → pydantic.v1.fields.ModelField). Confirm this is correct for the "v1 on v2" mode where pydantic v2 is installed but v1 compat layer is used.
• Verify the TypeAdapter usage (a pydantic v2 API) works correctly in the with_pydantic_v1_on_v2 context

Testing

• Seed snapshots updated for all affected fixtures
• All required CI checks pass (2 non-required checks fail due to pre-existing unrelated TypeScript compilation errors in @fern-api/ir-utils)

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