fix: prevent unsupported CN image input from breaking sessions

[CatStack-pixe]

Summary

• sanitize image parts for text-only DeepSeek/Zhipu models at the gateway boundary
• keep explicit vision models and non-CN providers unchanged
• cover HTTP and regular Codex WebSocket ingress with service tests

Verification

• git diff --check passed
• Go toolchain unavailable locally; GitHub Actions should run gofmt/tests

[CatStack-pixe]

Code audit: reviewed the five-file diff after CI. Image sanitization is limited to DeepSeek/Zhipu group platforms and text-only model names, preserves explicit vision models, recursively covers input and messages, and does not reuse image-generation permissions. HTTP mapping occurs before sanitization and the regular Codex WS ingress applies the same policy. CI passed for unit tests, integration tests, lint, security, frontend, and shell checks. No blocking findings.