fix: Transformer tags pydantic validation and shape compatibility

[wasim-builds]

Fixes #6090.

This PR fixes two issues with how tags are handled in Transformer.transform() in sagemaker-core:

1. It pops tags and Tags from the transformed dictionary before constructing the sagemaker.core.resources.TransformJob object. TransformJob does not expect or allow extra fields, so this prevents a pydantic.ValidationError (extra_forbidden).
2. It correctly formats tags and ensures capitalization (to Key and Value) before passing them in the CreateTransformJob API request. This prevents incompatibility when tags are passed in a format like [{"key": "foo", "value": "bar"}], which ModelTrainer accepts but backend APIs reject as invalid shape.

[wasim-builds]

By the way, I've really been enjoying contributing to this project! My goal is to get more involved in open source to help maintain projects and learn from great engineering teams. Is there a process or pathway for becoming a member of the aws GitHub organization? I'd love to continue helping out where I can!