fix: handle image_url rejection by retrying without images

Replace the static provider-level supports_vision check with a
reactive fallback: when a model returns an image-unsupported error,
strip image_url blocks from messages and retry once. This avoids
maintaining an inaccurate vision capability table and correctly
handles gateway/unknown model scenarios.

Also extract _safe_chat() to deduplicate try/except boilerplate
in chat_with_retry().

tests/test_provider_retry.py

@@ -123,3 +123,87 @@ async def test_chat_with_retry_explicit_override_beats_defaults() -> None:
     assert provider.last_kwargs["temperature"] == 0.9
     assert provider.last_kwargs["max_tokens"] == 9999
     assert provider.last_kwargs["reasoning_effort"] == "low"
+
+
+# ---------------------------------------------------------------------------
+# Image-unsupported fallback tests
+# ---------------------------------------------------------------------------
+
+_IMAGE_MSG = [
+    {"role": "user", "content": [
+        {"type": "text", "text": "describe this"},
+        {"type": "image_url", "image_url": {"url": "data:image/png;base64,abc"}},
+    ]},
+]
+
+
+@pytest.mark.asyncio
+async def test_image_unsupported_error_retries_without_images() -> None:
+    """If the model rejects image_url, retry once with images stripped."""
+    provider = ScriptedProvider([
+        LLMResponse(
+            content="Invalid content type. image_url is only supported by certain models",
+            finish_reason="error",
+        ),
+        LLMResponse(content="ok, no image"),
+    ])
+
+    response = await provider.chat_with_retry(messages=_IMAGE_MSG)
+
+    assert response.content == "ok, no image"
+    assert provider.calls == 2
+    msgs_on_retry = provider.last_kwargs["messages"]
+    for msg in msgs_on_retry:
+        content = msg.get("content")
+        if isinstance(content, list):
+            assert all(b.get("type") != "image_url" for b in content)
+            assert any("[image omitted]" in (b.get("text") or "") for b in content)
+
+
+@pytest.mark.asyncio
+async def test_image_unsupported_error_no_retry_without_image_content() -> None:
+    """If messages don't contain image_url blocks, don't retry on image error."""
+    provider = ScriptedProvider([
+        LLMResponse(
+            content="image_url is only supported by certain models",
+            finish_reason="error",
+        ),
+    ])
+
+    response = await provider.chat_with_retry(
+        messages=[{"role": "user", "content": "hello"}],
+    )
+
+    assert provider.calls == 1
+    assert response.finish_reason == "error"
+
+
+@pytest.mark.asyncio
+async def test_image_unsupported_fallback_returns_error_on_second_failure() -> None:
+    """If the image-stripped retry also fails, return that error."""
+    provider = ScriptedProvider([
+        LLMResponse(
+            content="does not support image input",
+            finish_reason="error",
+        ),
+        LLMResponse(content="some other error", finish_reason="error"),
+    ])
+
+    response = await provider.chat_with_retry(messages=_IMAGE_MSG)
+
+    assert provider.calls == 2
+    assert response.content == "some other error"
+    assert response.finish_reason == "error"
+
+
+@pytest.mark.asyncio
+async def test_non_image_error_does_not_trigger_image_fallback() -> None:
+    """Regular non-transient errors must not trigger image stripping."""
+    provider = ScriptedProvider([
+        LLMResponse(content="401 unauthorized", finish_reason="error"),
+    ])
+
+    response = await provider.chat_with_retry(messages=_IMAGE_MSG)
+
+    assert provider.calls == 1
+    assert response.content == "401 unauthorized"