feat: add Rust-Go interoperability suite

[muscariello]

Summary

• expand integrations/agntcy-a2a from a JSON-RPC smoke suite into a released-crate Rust <-> Go interoperability component covering JSON-RPC, HTTP+JSON, and gRPC
• switch the Rust fixture binaries to published agntcy-a2a 0.2.4, agntcy-a2a-client 0.1.9, agntcy-a2a-server 0.2.2, agntcy-a2a-grpc 0.1.6, and agntcy-a2a-pb 0.1.5
• add protocol-selectable Go and Rust fixtures, explicit gRPC transport registration in both client helpers, and per-transport Task targets / Ginkgo labels
• extend the reusable matrix to validate unary and streaming SendMessage, GetTask, ListTasks, CancelTask, negative-path error semantics, unsupported push behavior, and preservation of a mixed text plus structured-data request with message metadata through task history
• keep JSON-RPC and HTTP+JSON as full 4-leg pass/fail interop signals, and update the released Rust gRPC follow-up so the old endpoint mismatch is gone while one remaining Go-client incompatibility is still documented against the Rust gRPC ListTasks path
• update the component README and integration workflow to expose and run the full transport matrix in CI

Validation

• cargo build --manifest-path integrations/agntcy-a2a/fixtures/rust/Cargo.toml --bins
• PATH=/opt/homebrew/Cellar/go/1.26.1/libexec/bin:$PATH /opt/homebrew/Cellar/go/1.26.1/libexec/bin/go test . -count=1 -v -failfast -test.v -test.paniconexit0 -ginkgo.timeout 20m -timeout 20m -ginkgo.v -ginkgo.label-filter=grpc run from integrations/agntcy-a2a/tests

References

• Closes Validate A2A Rust <-> Go interoperability in CSIT #163
• Related upstream gap tracking: Align JSON-RPC and agent-card parsing with cross-SDK interoperability expectations a2a-rs#19
• Consumes the released HTTP+JSON follow-up from fix: align HTTP+JSON REST interop a2a-rs#21