Remove package:built_value dependency from HotReloadResponse

[jyameo]

Refactor HotReloadResponse to eliminate the dependency on package:built_value by using standard Dart JSON serialization instead.

Changes

• Rewrote HotReloadResponse as a simple Dart class with manual JSON serialization
• Replaced builder pattern constructor with standard named parameter constructor
• Implemented fromJson() factory and toJson() method for JSON serialization
• Maps JSON 'error' field to errorMessage property for clarity
• Removed HotReloadResponse from built_value serializers annotation
• Updated usage in client.dart to use new constructor syntax via the updated _sendResponse() pattern

Testing

Existing tests for hot reload functionality continue to pass with the updated implementation.

Fixes dart-lang/sdk#62289

[biggs0125]

Thanks!