feat(event-handler): add support for Pydantic Field discriminator in validation

[dap0am]

Summary

This PR adds support for Pydantic's Field(discriminator='...') syntax in event handler validation, enabling tagged unions to work correctly with the validation middleware.

Fixes #5953

Changes

• Enhanced get_field_info_annotated_type() in params.py to handle Field(discriminator) + Body() combinations
• Modified ModelField.__post_init__() in compat.py to preserve discriminator metadata when creating TypeAdapters
• Added comprehensive tests for discriminator validation and other Field features

Testing

• Added test_field_discriminator_validation() to verify discriminator functionality with tagged unions
• Added test_field_other_features_still_work() to ensure other Field features remain functional
• All existing tests pass without modification

Example Usage

from typing import Literal
from typing_extensions import Annotated
from pydantic import BaseModel, Field
from aws_lambda_powertools.event_handler import APIGatewayRestResolver
from aws_lambda_powertools.event_handler.openapi.params import Body

app = APIGatewayRestResolver(enable_validation=True)

class FooAction(BaseModel):
    action: Literal["foo"]
    foo_data: str

class BarAction(BaseModel):
    action: Literal["bar"]
    bar_data: int

# Now works with Field discriminator
Action = Annotated[FooAction | BarAction, Field(discriminator="action")]

@app.post("/actions")
def create_action(action: Annotated[Action, Body()]):
    return {"received": action.model_dump()}

Checklist

• Tests are passing
• Linting and type checking pass
• Follows project contribution guidelines
• Maintains backward compatibility

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[leandrodamascena]

Hey @dap0am please run make format to fix the CI.

[dap0am]

Hey @dap0am please run make format to fix the CI.

Done, thanks.

[leandrodamascena]

Hey @dap0am please run make format to fix the CI.

Done, thanks.

Thanks for fixing! You can run make pr before commit the code. So, you can see if there are errors in the code you refactored.

[leandrodamascena]

Hey @dap0am! Please let me know if you need any help here.

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud