[codex] Fix create_image follower RPC validation

[guowenlong123]

Summary

• validate create_image follower RPCs against their internal imageBase64 payload
• surface validation errors returned by the leader instead of only reporting HTTP 400
• add regression coverage for follower payload validation and error propagation

Root cause

The follower resolves the public source argument to imageBase64 before forwarding the request. The leader then validated that internal request with the public createImageInput schema, which still required source, so valid follower calls were rejected with HTTP 400.

Impact

create_image now works when the calling MCP process is a follower, and future leader-side validation errors remain actionable.

Validation

• npm test
• 2 tests passed
• TypeScript build passed

Fixes #35

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