fix: align jsonrpc and agent-card interop

[muscariello]

Fixes #19

Summary

• switch the Rust JSON-RPC method names to the spec-style CamelCase forms used by the A2A specification and sibling SDKs
• remove dotted method alias compatibility so unsupported legacy names fail with METHOD_NOT_FOUND
• treat null or missing AgentCard skills as an empty list for cross-SDK compatibility
• add regression coverage for canonical method validation, dotted-method rejection, and agent-card resolution

Validation

• cargo fmt --all --check
• cargo test -p agntcy-a2a -p agntcy-a2a-server -p agntcy-a2a-client

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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