Remove built_value dependency from ServiceExtensionRequest & ServiceExtensionResponse

[jyameo]

ServiceExtensionRequest:

This PR removes the package:built_value dependency from ServiceExtensionRequest and replaces it with standard Dart JSON serialization.

Changes

• Rewrote ServiceExtensionRequest as a simple Dart class with manual JSON serialization
• Removed ServiceExtensionRequest from the @SerializersFor annotation in serializers.dart
• Updated dev_handler.dart to serialize ServiceExtensionRequest using the wire format ['ServiceExtensionRequest', request.toJson()] in _serializeMessage()
• Added ServiceExtensionRequest case to _deserializeEvent() helper in client.dart to deserialize from wire format
• Changed constructor to use named parameters with a convenience fromArgs() factory method

Testing

• Existing tests for service extension functionality continue to pass with the updated implementation
• The change follows the same pattern used for HotReloadRequest, HotReloadResponse, HotRestartRequest, and HotRestartResponse

Fixes dart-lang/sdk#62302

---

ServiceExtensionResponse:

This PR removes the package:built_value dependency from ServiceExtensionResponse and replaces it with standard Dart JSON serialization.

Changes

• Rewrote ServiceExtensionResponse as a simple Dart class with manual JSON serialization
• Removed ServiceExtensionResponse from the @SerializersFor annotation in serializers.dart
• Updated client.dart to serialize ServiceExtensionResponse using the wire format ['ServiceExtensionResponse', response.toJson()] in _sendServiceExtensionResponse()
• Added ServiceExtensionResponse case to _deserializeMessage() helper in dev_handler.dart to deserialize from wire format
• Kept the fromResult() factory method for convenience and simplified the result field to be Map<String, dynamic>? instead of encoded JSON string

Testing

• Existing tests for service extension functionality continue to pass with the updated implementation
• The change follows the same pattern used for HotReloadRequest, HotReloadResponse, HotRestartRequest, and HotRestartResponse

Fixes dart-lang/sdk#62303