Add Pydantic support (PydanticClassDeclaration)

[mauriciogardini]

Summary

This branch adds Pydantic-oriented Python emitter support in @typespec/emitter-framework by aligning with the new APIs introduced in Alloy Python (PydanticClassDeclaration, richer pydantic/typing/pydantic_settings externals, and decorator ordering for class/static methods).

What changed

• Added a new emitter-framework wrapper:
  • python/components/class-declaration/PydanticClassDeclaration
  • Converts TypeSpec models to Pydantic classes (BaseModel default base)
  • Supports:
    • modelConfig (structured ConfigDict(...) generation)
    • modelConfigExpression (verbatim model_config = ... expression)
• Expanded Python builtins used by emitter-framework:
  • pydanticModule (including validators/serializers, ConfigDict, RootModel, subpaths)
  • typingModule (additional annotation symbols)
  • pydanticSettingsModule (BaseSettings, SettingsConfigDict)
• Added decorator support for typed class methods in emitter-framework:
  • Method now accepts decorators?: Children[]
  • Decorators render before intrinsic method decorators (@classmethod / @staticmethod) to support Pydantic validator stacking order.
• Updated Python test harness externals:
  • includes Pydantic and settings modules in externals.
• Added focused tests for new behavior:
  • pydantic-class-declaration.test.tsx
  • Covers pydantic model emission, model_config, validator ordering, and settings base usage.

Validation

• @typespec/emitter-framework build passes.
• Focused Python tests related to this work pass (including new Pydantic tests and class method decorator ordering tests).

Temporary dependency note

This branch currently points @alloy-js/python to the Alloy PR build (pkg.pr.new) so CI can consume the new APIs before Alloy is merged.

Remark: this dependency pin is temporary and should be reverted to the standard catalog/published Alloy dependency once Alloy PR branch changes are merged and released.