Add API changelog check workflow

[jonathan343]

Summary

Add a GitHub Actions workflow to ensure changelog entries are added when manually updating AWS models. When a PR modifies files in codegen/aws-models/, this workflow verifies that a corresponding changelog entry exists in clients/aws-sdk-<service>/.changes/next-release/.

Workflow Behavior

Trigger	Action
PR modifies codegen/aws-models/*.json	Checks for valid changelog entry
skip-changelog label present	Skips the check
Label added/removed	Re-runs to evaluate current state

Validation Requirements

A valid changelog entry must:

• Be a JSON file in clients/aws-sdk-/.changes/next-release/
• Contain "type": "api-change" and a "description" field

Test plan

• Open a PR that modifies a model file without a changelog entry → check fails
• Add a valid changelog entry → check passes
• Add skip-changelog label to a failing PR → check skips
• Remove skip-changelog label → check re-runs and validates

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[SamRemis]

Looks good over all - added some suggestions from Zizmor, which we should add to this repo to run over all GH actions to run by default in the long run.

[jonathan343]

Thanks @SamRemis! I just pushed a new commit with the fixes added from running uvx zizmor .github/workflows/api-changelog-check.yml --fix=all.

I opened this test PR to make sure this still runs properly.

Example: API model change with NO changelog entry:

Workflow: https://github.com/jonathan343/aws-sdk-python/actions/runs/20889335156/job/60017993475

Example: API model change with a changelog entry:

Workflow: https://github.com/jonathan343/aws-sdk-python/actions/runs/20889424472/job/60018217980