Umbrella issue to collect all scheduled API breaking changes which can be done with 1.0 release.

Note: this issue doesn't track 1.0 support itself, for this see #321.

• A2AClient - this client is built around JSON-RPC types and can't be used in transport-agnostic way. New transport agnostic client can be renamed to A2AClient or this one can just be removed.
• JsonRpcTransport - remove all the code created for compatibility with the old A2AClient client.
• Follow-up for fix: export transport agnostic errors from client #272 - remove legacy JSON-RPC errors from src/index.ts and export transport agnostic errors instead.
• A2AExpressApp - should be removed in favor of using middlewares directly, original refactoring was done in refactor: extract request handlers from A2AExpressApp #162.
• Drop Node 18 support - this version is no longer maintained (re [Feat]: Update node verison used in CI #165, ci: run tests against maintained Node LTS versions #256).
• ServerCallContext - the parameter should be made mandatory in all extension points.
• Client transports - The client transports are exported via index.js, we should assess if it is really needed
• Expose protocolName from Transport.