remove built_value dependency from DevToolsRequest/DevToolsResponse

[jyameo]

DevToolsRequest/DevToolsResponse

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

Changes

• Rewrote DevToolsRequest and DevToolsResponse as simple Dart classes with manual JSON serialization
• Removed DevToolsRequest and DevToolsResponse from the @SerializersFor annotation in serializers.dart
• Updated serialization logic in dev_handler.dart to use the wire format ['DevToolsRequest', json] and ['DevToolsResponse', json]
• Updated extension_debugger.dart to manually deserialize DevToolsRequest
• Updated client-side logic in client.dart to use named parameters and toJson() for serialization
• Updated debug_session.dart in the extension to handle the new class structure
• Removed generated file devtools_request.g.dart

Testing

• Existing tests for devtool requests continue to pass with the updated implementation
• Follows the same pattern used for DebugEvent, ConnectRequest, RunRequest, BuildResult, and other RPC classes

Fixes dart-lang/sdk#62296

[auto-submit]

autosubmit label was removed for dart-lang/webdev/2742, because Pull request dart-lang/webdev/2742 is not in a mergeable state.

[bkonyi]

Do these need to be non-nullable?

[bkonyi]

This isn't the greatest way to calculate a hash code. Consider using Object.hash(...) instead.

[bkonyi]

Same here.

[bkonyi]

This can use object destructuring:

if (decoded case ['DevToolsRequest', final Map<String, Object?> request]) {
  return DevToolsRequest.fromJson(request);
}