feat: add SLIMRPC transport binding

[muscariello]

Summary

• add a new a2a-slimrpc workspace crate for SLIMRPC client and server bindings
• add the SLIMRPC transport protocol constant and workspace/documentation plumbing
• bridge A2A request handling, protobuf conversions, and SLIMRPC error mapping

Testing

• cargo test -p agntcy-a2a-slimrpc
• cargo check --workspace

Closes #13
Part of #12

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.01518% with 21 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
a2a-slimrpc/src/server.rs	90.62%	18 Missing ⚠️
a2a-slimrpc/src/client.rs	98.03%	3 Missing ⚠️

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