Fix GPT Image generation requests with unsupported response format

[artcc]

Summary

• Omit response_format from dedicated image generation requests for GPT Image and aliased/unknown models, while retaining b64_json for explicit DALL·E 2 and 3 IDs.
• Cover the serialized requests and URL response fallback with focused unit tests; update the image API specification and release notes for 1.7.15 (build 117).
• Stabilize the conversation-list loading test by observing the models-loaded state instead of relying on a fixed number of yields.

Review note

• This branch also raises the iOS and macOS deployment targets from 26.0 to 26.6 across the Xcode project; please review that change before merging.

Verification

• Changes reviewed statically. Tests and builds were not run in this environment.

Refs #129