fix: preserve ANY routes when merging API definitions

[proton0210]

Issue #, if available

Closes #4000. Thanks to @tiagohconte for the reproduction and proposed fix.

Description of changes

With MergeDefinitions: true, SAM writes ANY events under any instead of x-amazon-apigateway-any-method. It also drops generated routes when the inline paths is empty, null or missing.

Use the normalized method name for both lookup and write-back, and retain the paths dictionary in the merged body. Existing inline fields are preserved while SAM's generated integration takes precedence. Correcting the API body changes the deployment hash for affected templates.

Description of how you validated changes

• make pr on Python 3.12: all checks passed, 4,672 tests passed, 95.63% coverage.
• Eight new regression cases fail before the fix and pass afterward; a GET control passes in both versions.
• Transform snapshots cover missing/null paths and an existing OpenAPI ANY operation with CORS in all three partitions.
• An additional local audit passed 60 transform combinations covering method casing, Swagger/OpenAPI, five paths states, CORS and multiple events.

No live AWS deployment was run.

Checklist

• Review the generative AI contribution guidelines
• Adheres to the development guidelines
• Add/update transform tests
  • Using correct values
  • Using wrong values (unit regression for an invalid inline ANY operation)
• Add/update integration tests — covered by local transform tests; no deployed integration test added.

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