Implement full A2A Server support

[bdmorgan]

What would you like to be added?

An A2A server implementation was recently added to the Gemini CLI repository in packages/a2a-server. However, this server was originally developed in a separate repository, gemini-cli-a2a, for a specific use case: facilitating communication with IDE extensions (primarily the VSCode extension). While it serves this purpose effectively, its protocol is only "A2A-like" and does not fully adhere to the formal Agent-to-Agent (A2A) specification. Furthermore, its implementation began before the a2a-js library actually existed which has resulted in some inconsistencies.

Why is this needed?

Important for supporting "CLI Everywhere" initiative - see go/gcli-cr-a2a-server-design for more information

Additional context

No response