refactor: move document extraction from ContextBuilder to API layer

ContextBuilder._build_user_content now only handles images (its original
responsibility).  Document text extraction (PDF, DOCX, XLSX, PPTX) is
performed by the new _extract_documents() helper in server.py, called
before process_direct().  This keeps the core context builder free of
format-specific dependencies and makes the API boundary the single place
where uploaded files are pre-processed.

Tests updated to reflect the new responsibility boundary.

Made-with: Cursor

tests/test_api_attachment.py

@@ -10,6 +10,7 @@ import pytest
 import pytest_asyncio
 
 from nanobot.api.server import (
+    _extract_documents,
     _FileSizeExceeded,
     _parse_json_content,
     _save_base64_data_url,
@@ -184,7 +185,7 @@ def test_parse_json_content_rejects_oversized_base64_file(tmp_path) -> None:
 @pytest.mark.skipif(not HAS_AIOHTTP, reason="aiohttp not installed")
 @pytest.mark.asyncio
 async def test_multipart_upload_saves_file(aiohttp_client, mock_agent, tmp_path) -> None:
-    """Multipart upload saves file to media dir and passes path to process_direct."""
+    """Multipart upload of non-image extracts text into content (not media)."""
     import os
     original_cwd = os.getcwd()
     os.chdir(tmp_path)
@@ -202,8 +203,9 @@ async def test_multipart_upload_saves_file(aiohttp_client, mock_agent, tmp_path)
         )
         assert resp.status == 200
         call_kwargs = mock_agent.process_direct.call_args.kwargs
-        assert call_kwargs["content"] == "analyze this"
-        assert len(call_kwargs.get("media", [])) == 1
+        assert "analyze this" in call_kwargs["content"]
+        # Non-image file text is extracted into content, not kept as media
+        assert not call_kwargs.get("media")
     finally:
         os.chdir(original_cwd)
 
@@ -371,13 +373,62 @@ async def test_json_base64_image_upload(aiohttp_client, mock_agent, tmp_path) ->
 
 
 # ---------------------------------------------------------------------------
-# DOCX document extraction tests
+# _extract_documents tests (API-layer document extraction)
+# ---------------------------------------------------------------------------
+
+def test_extract_documents_separates_images_from_docs(tmp_path) -> None:
+    """Images stay in media; document text is appended to content."""
+    from docx import Document
+
+    png = tmp_path / "chart.png"
+    png.write_bytes(b"\x89PNG\r\n\x1a\n" + b"\x00" * 100)
+
+    doc = Document()
+    doc.add_paragraph("Quarterly revenue is $5M")
+    docx_path = tmp_path / "report.docx"
+    doc.save(docx_path)
+
+    text, image_paths = _extract_documents("summarize", [str(png), str(docx_path)])
+    assert len(image_paths) == 1
+    assert image_paths[0] == str(png)
+    assert "Quarterly revenue" in text
+    assert "summarize" in text
+
+
+def test_extract_documents_skips_extraction_errors(tmp_path, monkeypatch) -> None:
+    """Document extraction errors should not leak into user text."""
+    bad_file = tmp_path / "broken.docx"
+    bad_file.write_text("not a docx", encoding="utf-8")
+
+    import nanobot.api.server as _srv
+    monkeypatch.setattr(
+        _srv, "extract_text",
+        lambda _path: "[error: failed to extract DOCX: boom]",
+    )
+
+    text, image_paths = _extract_documents("hello", [str(bad_file)])
+    assert text == "hello"
+    assert image_paths == []
+
+
+def test_extract_documents_images_only(tmp_path) -> None:
+    """When all files are images, text is unchanged and all paths kept."""
+    png = tmp_path / "a.png"
+    png.write_bytes(b"\x89PNG\r\n\x1a\n" + b"\x00" * 100)
+    text, image_paths = _extract_documents("describe", [str(png)])
+    assert text == "describe"
+    assert len(image_paths) == 1
+
+
+# ---------------------------------------------------------------------------
+# DOCX end-to-end upload test (API layer now extracts text)
 # ---------------------------------------------------------------------------
 
 @pytest.mark.skipif(not HAS_AIOHTTP, reason="aiohttp not installed")
 @pytest.mark.asyncio
 async def test_docx_upload_extracted_and_sent(aiohttp_client, tmp_path) -> None:
-    """Uploaded DOCX should have its text extracted before being sent to AI."""
+    """Uploaded DOCX text should be extracted at the API layer and
+    appended to the content string, not passed as media."""
     from docx import Document
 
     agent = _make_mock_agent("This report shows $5M revenue")
@@ -405,8 +456,9 @@ async def test_docx_upload_extracted_and_sent(aiohttp_client, tmp_path) -> None:
         resp = await client.post("/v1/chat/completions", data=data)
         assert resp.status == 200
         call_kwargs = agent.process_direct.call_args.kwargs
-        media = call_kwargs.get("media", [])
-        assert len(media) == 1
-        assert "report.docx" in media[0]
+        # Document text should be extracted into content, not media
+        assert "Total revenue" in call_kwargs["content"]
+        # No media (docx is not an image)
+        assert not call_kwargs.get("media")
     finally:
         os.chdir(original_cwd)