Use image-generation output_format for the result media type

[PratikDhanave (PratikDhanave)]

The Responses image-generation result media type was hardcoded to png on two paths — imageGenerationContents (non-streaming) and the output_item.done non-partial fallback — so an image generated as webp/jpeg was surfaced as image/png. The streaming partial path already does this correctly with cmp.Or(event.OutputFormat, "png").

Python derives the media type from the image (detect_media_type_from_base64(...) or "image/png"); the call also carries output_format.

Change

• Replace the two "png" literals with cmp.Or(item.OutputFormat, "png") (ResponseOutputItemImageGenerationCall.OutputFormat), matching the partial path.

Test

• TestResponsesImageGenerationCall_UsesOutputFormat: an image_generation_call with "output_format":"webp" now yields image/webp. Fails before (image/png), passes after.

[Copilot]

Copilot review overview

🟡 Changes recommended

The streaming fallback path lacks regression coverage for non-PNG output formats.

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Review effort: Lite
Findings: 1

Open (1)

• Test missing webp media type for streaming fallback · New

What changed in this PR

Updates image-generation result media types to honor output_format instead of defaulting to PNG.

Changes:

• Uses OutputFormat for non-streaming and streaming fallback results.
• Adds WebP regression coverage for non-streaming responses.

File	Summary
provider/​openaiprovider/​responses.go	Derives image media types from output format.
provider/​openaiprovider/​responses_test.go	Tests WebP media type handling.

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[github-actions]

Scope: user-visible behavior (internal implementation, but observable via message.DataContent.MediaType)
Changed Go contract: None (no exported functions/types/signatures changed). The unexported imageGenerationResult/imageGenerationContents helpers in provider/openaiprovider/responses.go now use cmp.Or(item.OutputFormat, "png") instead of a hardcoded "png" literal on two paths (non-streaming imageGenerationContents and the streaming output_item.done non-partial fallback), affecting the value of the pre-existing public message.DataContent.MediaType field returned to callers.
Upstream evidence reviewed: python/packages/openai/agent_framework_openai/_chat_client.py — _image_generation_item_to_contents (non-streaming/output_item.done fallback path, ~L2337) uses detect_media_type_from_base64(data_str=image_result) or 'image/png'; the response.image_generation_call.partial_image streaming handler (~L3707) uses the same detect_media_type_from_base64(...) or 'image/png' pattern. python/packages/core/agent_framework/_types.py::detect_media_type_from_base64 (L83) is the shared helper. Both Python paths already derive the real media type instead of hardcoding png.
Result: aligned. This PR brings the Go non-streaming and non-partial streaming paths in line with the behavior the streaming partial path already had (cmp.Or(event.OutputFormat, "png"), responses.go L1557), and matches Python's intent of surfacing the actual image format with a png fallback rather than always reporting png. Go derives the format from the API-provided output_format field rather than sniffing base64 magic bytes as Python does, but both achieve the same observable contract: correct media type when known, png otherwise. No exported API changed and no cross-repo divergence found; this is a straightforward parity bugfix with test coverage (TestResponsesImageGenerationCall_UsesOutputFormat, TestResponsesStreamingImageGenerationCall_NoPartial_UsesOutputFormat).

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