Enable user verification for local Codex Desktop sessions

[copyberry]

Enable user verification for local Codex Desktop sessions

What changed

Allow app-server to advertise openai/elicitation.userVerification and route verification requests to local stdio clients named Codex Desktop when experimentalApi is enabled and the device supports verification. Preserve support for the in-process codex-tui client.

Document the experimental verification API, local credential lifecycle, transport restrictions, and GUI requirements for cancellation and late proofs.

Testing

Extend activation tests to cover desktop capability advertisement and request eligibility, including rejection for other transports, unrecognized client names, missing opt-in, and unsupported devices.

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