feat: use ProtoJSON for JSON wire payloads

[muscariello]

Summary

• make a2a.proto the wire-format source of truth for JSON payload bodies by generating ProtoJSON-capable types in a2a-pb
• add a shared a2a_pb::protojson_conv bridge between native Rust models and protobuf/ProtoJSON payloads
• move JSON-RPC and REST request, response, and streaming payload serialization onto ProtoJSON while keeping the handwritten JSON-RPC envelope and REST path/query handling
• accept null repeated and map fields from generated ProtoJSON serde as empty collections for cross-SDK compatibility
• accept bare host:port gRPC endpoints by normalizing them to http://... before connecting so cross-SDK agent cards remain usable

Testing

• cargo test -p agntcy-a2a-pb -p agntcy-a2a-client -p agntcy-a2a-server -p agntcy-a2a-grpc

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.14176% with 44 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
a2a-client/src/rest.rs	57.89%	16 Missing ⚠️
a2a-server/src/rest.rs	70.83%	14 Missing ⚠️
a2a-client/src/jsonrpc.rs	82.50%	7 Missing ⚠️
a2a-pb/src/protojson_conv.rs	94.23%	3 Missing ⚠️
a2a-grpc/src/client.rs	91.30%	2 Missing ⚠️
a2a-server/src/jsonrpc.rs	96.66%	2 Missing ⚠️

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