fix(openai-shim): handle image tool results per provider

[stamsam]

Summary

• add provider-gated handling for image content returned inside tool_result blocks
• keep OpenAI default behavior unchanged
• make DeepSeek tool-result images text-only so sessions do not 400 on image_url
• split Xiaomi MiMo/OpenGateway image tool results into tool text plus a user image message

Fixes #425.

Testing

• bun test src/services/api/openaiShim.test.ts -t "DeepSeek image tool results"
• bun test src/services/api/openaiShim.test.ts -t "Xiaomi MiMo image tool results"
• bun test src/services/api/openaiShim.test.ts -t "preserves image tool results"
• bun test src/services/api/openaiShim.test.ts -t "preserves mixed text and image tool results"
• git diff --check

[jatmn]

Closing this as a duplicate/competing follow-up to the earlier OpenAI-shim image tool-result work in #237, with related overlap in #268.

#237 predates this PR and already covers preserving images in tool_result handling in src/services/api/openaiShim.ts with focused tests. That existing PR is already under maintainer review, including a current requested-change thread about how image tool results should be represented safely for provider compatibility. #268 also overlaps the same tool-result image handling area while addressing third-party request leakage.

Please continue the fix on the existing reviewed PR thread rather than opening a parallel branch for the same shim behavior.