feat(acp): pass image file attachments through as image_url parts

Extends PR #21400's resource inlining with image-specific handling: ACP
resource_link and embedded blob resources with an image/* mime (or image
file suffix when mime is missing) now emit an OpenAI image_url part
with a base64 data URL, so vision models actually see the image
instead of a [Binary file omitted] note. Non-image resources keep the
existing text-inlining behavior.

Adds 3 tests: local PNG via resource_link, JPEG mime inferred from
suffix when client omits mimeType, and embedded blob PNG.

tests/acp_adapter/test_acp_images.py

@@ -1,5 +1,8 @@
+import base64
+
 import pytest
 from acp.schema import (
+    BlobResourceContents,
     EmbeddedResourceContentBlock,
     ImageContentBlock,
     ResourceContentBlock,
@@ -82,3 +85,75 @@ async def test_initialize_advertises_image_prompt_capability():
     assert response.agent_capabilities is not None
     assert response.agent_capabilities.prompt_capabilities is not None
     assert response.agent_capabilities.prompt_capabilities.image is True
+
+
+# 1x1 transparent PNG — smallest valid image payload for inlining tests.
+_ONE_PX_PNG = bytes.fromhex(
+    "89504e470d0a1a0a0000000d49484452000000010000000108060000001f15c4"
+    "890000000a49444154789c6300010000000500010d0a2db40000000049454e44ae426082"
+)
+
+
+def test_acp_resource_link_image_file_is_inlined_as_image_url(tmp_path):
+    attached = tmp_path / "shot.png"
+    attached.write_bytes(_ONE_PX_PNG)
+
+    content = _content_blocks_to_openai_user_content([
+        TextContentBlock(type="text", text="Look at this screenshot"),
+        ResourceContentBlock(
+            type="resource_link",
+            name="shot.png",
+            uri=attached.as_uri(),
+            mimeType="image/png",
+        ),
+    ])
+
+    assert isinstance(content, list)
+    # [user text, image header, image_url]
+    assert content[0] == {"type": "text", "text": "Look at this screenshot"}
+    assert content[1]["type"] == "text"
+    assert "[Attached image: shot.png]" in content[1]["text"]
+    assert content[2]["type"] == "image_url"
+    expected_url = "data:image/png;base64," + base64.b64encode(_ONE_PX_PNG).decode("ascii")
+    assert content[2]["image_url"]["url"] == expected_url
+
+
+def test_acp_resource_link_image_mime_inferred_from_suffix(tmp_path):
+    """No mimeType sent — should still be recognised as image by file suffix."""
+    attached = tmp_path / "pic.jpg"
+    attached.write_bytes(_ONE_PX_PNG)  # content doesn't matter for the code path
+
+    content = _content_blocks_to_openai_user_content([
+        ResourceContentBlock(
+            type="resource_link",
+            name="pic.jpg",
+            uri=attached.as_uri(),
+        ),
+    ])
+
+    assert isinstance(content, list)
+    image_parts = [p for p in content if p.get("type") == "image_url"]
+    assert len(image_parts) == 1
+    assert image_parts[0]["image_url"]["url"].startswith("data:image/jpeg;base64,")
+
+
+def test_acp_embedded_blob_image_is_inlined_as_image_url():
+    b64 = base64.b64encode(_ONE_PX_PNG).decode("ascii")
+    content = _content_blocks_to_openai_user_content([
+        EmbeddedResourceContentBlock(
+            type="resource",
+            resource=BlobResourceContents(
+                uri="file:///tmp/embed.png",
+                mimeType="image/png",
+                blob=b64,
+            ),
+        ),
+    ])
+
+    assert isinstance(content, list)
+    assert content[0]["type"] == "text"
+    assert "[Attached image: embed.png]" in content[0]["text"]
+    assert content[1] == {
+        "type": "image_url",
+        "image_url": {"url": f"data:image/png;base64,{b64}"},
+    }