feat: add proto2pydantic Pydantic model generation pipeline

Add protoc-gen-proto2pydantic plugin to buf.gen.yaml for generating
Pydantic models alongside existing protobuf _pb2 objects.

Generated models:
- 39 Pydantic models + 2 enums from a2a.proto
- All extend A2ABaseModel with to_proto_json() for ProtoJSON compat
- Keyword escaping (list -> list_ with alias='list')
- Timestamp field serializers for RFC 3339
- Oneof unions for Part.content, SecurityScheme.scheme, etc.

This is an additive change - existing a2a_pb2 imports are untouched.
New Pydantic models available via: from a2a.types.a2a_pydantic import ...

Author: brucearctor

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