fix: support LocalStack-compatible _user_request_ URL for API Gateway execution

[hectorvent]

Summary

Closes #295

Problem

Requests to the LocalStack-compatible execute URL format:

  GET /restapis/{apiId}/{stageName}/_user_request_/{proxy+}                                                                                                                 

had no handler in Floci. The request fell through to S3's wildcard
handler and returned a misleading NoSuchBucket XML error.

Verified manually with a Python Lambda + AWS CLI against a running. Floci container. Both formats return identical responses:

  curl http://localhost:4566/execute-api/{apiId}/prod/hello                                                                                                                 
  curl http://localhost:4566/restapis/{apiId}/prod/_user_request_/hello

Type of change

• Bug fix (fix:)
• New feature (feat:)
• Breaking change (feat!: or fix!:)
• Docs / chore

AWS Compatibility

Checklist

• ./mvnw test passes locally
• New or updated integration test added
• Commit messages follow Conventional Commits