[Remote Agents] OAuth 2.0: Authorization Code Flow

[adamfweidman]

Implement support for the OAuth 2.0 Authorization Code flow for Remote Agents.

Requirements:

• Adapt the existing MCPOAuthProvider loopback server for A2A interactive authentication.
• Support standard authorization and token exchange logic.
• Ensure secure token storage and refresh handling.

https://docs.google.com/document/d/1szEaFst37OFPV_IKJYSLTNODM6lRrgWk_WuN2okpTtY/edit?tab=t.0 contains all workspace testing instructions

[Ujjiyara]

/assign

[gemini-cli]

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