feat: migrate to Pydantic v2

[bnusunny]

migrate to Pydantic v2

• Update pydantic dependency from v1 to v2
• Remove v1/v2 compatibility shim in samtranslator/compat.py
• Migrate deprecated APIs:
  • parse_obj() -> model_validate()
  • schema() -> model_json_schema()
  • dict() -> model_dump()
  • root -> RootModel with .root accessor
  • class Config -> model_config = ConfigDict()
• Update Field() to use json_schema_extra for custom schema properties
• Add explicit default values for all Optional fields
• Add type aliases to avoid field name shadowing in Pydantic v2
• Update JSON schema generation to normalize $defs -> definitions
• Upgrade schema version from draft-04 to draft-07
• Update validation error handling for v2 error format
• Update mypy to >=1.5.0 for Pydantic v2 plugin compatibility
• Add hypothesis test dependency

Checklist

• Review the generative AI contribution guidelines
• Adheres to the development guidelines
• Add/update transform tests
  • Using correct values
  • Using wrong values
• Add/update integration tests

Examples?

Please reach out in the comments if you want to add an example. Examples will be
added to sam init through aws/aws-sam-cli-app-templates.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.