Which issue(s) does this change fix?

#1877

Why is this change necessary?

sam local generate-event only supports string values for parameters. When you need to pass complex data structures like queryStringParameters for API Gateway events, you have to manually edit the generated JSON file afterward.

How does it address the issue?

The implementation uses a placeholder strategy to safely handle dictionary values. Before template rendering, dict-type parameters are replaced with unique placeholders to avoid JSON escaping issues. After Chevron renders the template, the placeholders are replaced with actual parsed dictionary objects. This allows users to pass JSON strings via CLI that get converted to proper dict objects in the final event output.

What side effects does this change have?

This change is fully backward compatible with no breaking changes. Existing string parameters continue to work unchanged. The only side effect is a minor performance overhead from parsing JSON strings, which is negligible. Invalid JSON strings are silently kept as strings rather than throwing errors, which could potentially mask user input mistakes but maintains compatibility.

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

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