feat: add proto2pydantic Pydantic model generation pipeline

buf.gen.yaml

@@ -29,3 +29,10 @@ plugins:
   # Generates *_pb2.pyi files.
   - remote: buf.build/protocolbuffers/pyi
     out: src/a2a/types
+  # Generate Pydantic models with ProtoJSON compatibility
+  - local: protoc-gen-proto2pydantic
+    out: src/a2a/types
+    opt:
+      - preset=a2a
+      - base_class=a2a._base.A2ABaseModel
+      - output_file=a2a_pydantic.py

scripts/gen_proto.sh

@@ -1,6 +1,10 @@
 #!/bin/bash
 set -e
 
+# Install proto2pydantic plugin for Pydantic model generation
+echo "Installing protoc-gen-proto2pydantic@v0.4.0..."
+go install github.com/protocgen/proto2pydantic@v0.4.0
+
 # Run buf generate to regenerate protobuf code and OpenAPI spec
 npx --yes @bufbuild/buf generate