[bug] Responses codec emits adjacently-tagged ImageSource/FileSource into image_url/file

Symptom

The Responses codec serializes the adjacently-tagged ImageSource enum directly into image_url, emitting an object where the Responses API requires a bare URL or data-URI string. The same defect applies to FileSource in input_file. Any image or file translated into the Responses format is unreadable upstream.

Cause

crates/switchyard-translation/src/codecs/responses/buffered.rs (in encode_responses_content):

ContentBlock::Image { source } => {
    blocks.push(json!({"type": "input_image", "image_url": source}));
}
...
ContentBlock::File { source } => {
    blocks.push(json!({"type": "input_file", "file": source}));
}

ImageSource and FileSource are adjacently tagged (crates/protocol/src/llm.rs):

#[serde(tag = "type", content = "data", rename_all = "snake_case")]

so the emitted item is

{"type": "input_image",
 "image_url": {"type": "url", "data": {"url": "https://…", "detail": null}}}

Expected vs. actual

• Expected: "image_url": "https://…" — a string, as the API requires and as the decoder reads back.
• Actual: a nested tagged object; upstream cannot read it.

⭐ The asymmetry is the clearest evidence this is unintended: all three codecs encode the same ImageSource, and only this one serializes it raw.

Codec	Handling
openai_chat/buffered.rs	openai_image_part(source) — builds {"url": …} explicitly ✅
anthropic/buffered.rs	match source { … } — destructures ✅
responses/buffered.rs	serializes the enum inline ⛔

And within the same match block in the Responses codec, Audio and Video are destructured correctly — only Image and File are not.

Why it survived the test suite

crates/switchyard-translation/tests/lossless_roundtrip.rs uses the Chat shape for the Responses fixture:

{"type": "input_image", "image_url": {"url": "https://example.test/image.png", "detail": "high"}}

decode_image_source accepts both an object and a string, so the fixture round-trips while never pinning the shape the encoder must emit. Real clients send "image_url": "data:image/png;base64,…" as a string.

Reproduction

Translate an Anthropic request carrying a base64 image to openai_responses and inspect input[0].content. The added test anthropic_image_encodes_as_responses_input_image_string fails on main and passes with the fix.

⚠ Note this is not reached on a same-format Responses→Responses route, because encode_request short-circuits to exact_preserved_request and replays the preserved body verbatim. It bites on cross-format routes.

Environment

• Commit: 7f3b2fe9 (main); introduced in the initial commit 86020fab, so present in v0.2.0 too.

Suggested fix

Destructure both enums into the Responses wire shape, mirroring openai_image_part, and record a lossy diagnostic instead of dropping an unmappable source silently. PR: see linked pull request.

Start in crates/switchyard-translation/src/codecs/responses/buffered.rs at encode_responses_content, then compare the image handling in openai_chat/buffered.rs and anthropic/buffered.rs. Run the added anthropic_image_encodes_as_responses_input_image_string test and inspect the Responses fixtures in crates/switchyard-translation/tests/lossless_roundtrip.rs. Done means image_url and file use the required Responses wire shapes and unmappable sources produce a lossy diagnostic.