Launch image generation by default (#17153)

## Summary
- Promote `image_generation` from under-development to stable
- Enable image generation by default in the feature registry
- Update feature coverage for the new launch-state expectation
- Add the missing image-generation auth fixture field in a tool registry
test

## Testing
- `just fmt`
- `cargo test -p codex-features`
- `cargo test -p codex-tools` currently fails:
`test_full_toolset_specs_for_gpt5_codex_unified_exec_web_search` needs
its expected default tool list updated for `image_generation`

codex-rs/tools/src/tool_registry_plan_tests.rs

@@ -97,6 +97,7 @@ fn test_full_toolset_specs_for_gpt5_codex_unified_exec_web_search() {
             search_context_size: None,
             search_content_types: None,
         },
+        create_image_generation_tool("png"),
         create_view_image_tool(ViewImageToolOptions {
             can_request_original_image_detail: config.can_request_original_image_detail,
         }),