fix(client): serialize create-push requests for go interop

[muscariello]

Summary

• serialize JSON-RPC create-push requests with the nested SDK request shape expected by a2a-go
• keep the existing ProtoJSON path for other request types and reuse the generic JSON-RPC result handling
• add a focused compatibility test covering the nested create-push request payload

Validation

• cargo test -p agntcy-a2a-client
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.65272% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
a2a-client/src/jsonrpc.rs	97.11%	6 Missing ⚠️
a2a-client/src/push_config_compat.rs	93.54%	2 Missing ⚠️

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