build: fix release publication flow

[muscariello]

Summary

• fix CI failures from clippy and formatting issues
• move cross-crate transport E2E coverage into the non-publishable example crate
• switch published package names to the agntcy namespace and configure release-plz for gated first-time publishing

Why

• unblocks the failing release workflow on main
• avoids crates.io package-name collisions for a2a and a2a-client
• keeps the existing Rust import paths unchanged for consumers

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

Unblocks #4.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.29412% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
a2a-grpc/src/server.rs	66.66%	3 Missing ⚠️
a2a-client/src/client.rs	0.00%	2 Missing ⚠️

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