feat: support RFC 7396 JSON Merge Patch on PATCH endpoints

[chiemezie1]

feat: support RFC 7396 JSON Merge Patch on PATCH endpoints

Summary

• Added support for JSON Merge Patch (RFC 7396) on subscription and plan PATCH endpoints.
• Implemented partial update handling for merge-patch payloads, including explicit null values to clear fields and missing fields to leave values unchanged.
• Rejected unknown fields with a 400 response to keep the payload contract strict and secure.
• Added regression tests and updated OpenAPI documentation to advertise the new content type.

Testing

• Verified the validator package builds successfully:
  • go test ./internal/validator
• Verified the updated OpenAPI YAML parses successfully:
  • Python YAML parse of openapi.yaml

Closes: #404

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