fix(interop): align push-config JSON bindings with a2a-go

[muscariello]

Summary

• align JSON-RPC and REST push-config responses with the JSON shapes expected by a2a-go
• keep Rust JSON-RPC and REST clients backward-compatible with the older ProtoJSON push-config payloads during rollout
• return HTTP 200 for REST push-config deletes and cover the positive delete path in server tests

Validation

• cargo fmt --all -- --check
• cargo test -p agntcy-a2a-server
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.13260% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
a2a-client/src/push_config_compat.rs	94.73%	4 Missing ⚠️
a2a-client/src/rest.rs	93.10%	2 Missing ⚠️
a2a-server/src/rest.rs	97.87%	1 Missing ⚠️

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