fix: send images to Mistral as base64 vision chunks instead of placeholders

The text provider now includes reference_images alongside inputs when
building prompts. Image files are sent as base64 data URLs via
ImageURLChunk for actual multimodal vision support, replacing the
previous [Attached image: ...] placeholder text.

tests/test_providers.py

@@ -2,7 +2,7 @@
 
 Mock-heavy tests produce many Any-typed expressions from MagicMock.
 """
-# pyright: reportAny=false
+# pyright: reportAny=false, reportUnknownMemberType=false
 
 from __future__ import annotations
 
@@ -312,8 +312,10 @@ class TestTextProvider:
 
             call_args = mock_client.chat.complete_async.call_args
             messages = call_args.kwargs["messages"]
-            prompt_text = messages[0].content
-            assert "[Attached image: photo.png]" in prompt_text
+            chunks = messages[0].content
+            assert isinstance(chunks, list)
+            assert chunks[0].text == "Describe this image"
+            assert chunks[1].image_url.url.startswith("data:image/png;base64,")
 
     async def test_text_no_choices_raises(self, project_dir: Path) -> None:
         target_config = TargetConfig(prompt="x")
@@ -374,9 +376,38 @@ class TestTextProvider:
             )
 
             call_args = mock_client.chat.complete_async.call_args
-            prompt_text = call_args.kwargs["messages"][0].content
-            assert "--- Contents of a.txt ---" in prompt_text
-            assert "content A" in prompt_text
-            assert "--- Contents of b.txt ---" in prompt_text
-            assert "content B" in prompt_text
-            assert "[Attached image: c.png]" in prompt_text
+            chunks = call_args.kwargs["messages"][0].content
+            assert isinstance(chunks, list)
+            # TextChunk for prompt, TextChunk for a.txt, TextChunk for b.txt,
+            # ImageURLChunk for c.png
+            assert chunks[0].text == "Combine all"
+            assert "content A" in chunks[1].text
+            assert "content B" in chunks[2].text
+            assert chunks[3].image_url.url.startswith("data:image/png;base64,")
+
+    async def test_text_with_reference_images(self, project_dir: Path) -> None:
+        _ = (project_dir / "ref.png").write_bytes(b"\x89PNG")
+
+        target_config = TargetConfig(
+            prompt="Describe the style", reference_images=["ref.png"]
+        )
+        response = _make_text_response("A stylized image")
+
+        with patch("bulkgen.providers.text.Mistral") as mock_cls:
+            mock_client = _make_mistral_mock(response)
+            mock_cls.return_value = mock_client
+
+            provider = TextProvider(api_key="test-key")
+            await provider.generate(
+                target_name="desc.txt",
+                target_config=target_config,
+                resolved_prompt="Describe the style",
+                resolved_model="mistral-large-latest",
+                project_dir=project_dir,
+            )
+
+            call_args = mock_client.chat.complete_async.call_args
+            chunks = call_args.kwargs["messages"][0].content
+            assert isinstance(chunks, list)
+            assert chunks[0].text == "Describe the style"
+            assert chunks[1].image_url.url.startswith("data:image/png;base64,")