test(converter): guard that importing converter stays markitdown-free

Adds a regression test that spawns a fresh interpreter and asserts
`import openkb.converter` loads none of markitdown / magika / onnxruntime.
This locks in the lazy-import optimization: if a future change reintroduces a
module-level markitdown import, startup cost and the slim-packaging benefit
regress silently while every functional test still passes. The check runs in a
subprocess so it is deterministic regardless of what earlier tests imported
into this process's sys.modules.

Claude-Session: https://claude.ai/code/session_01KZyUSGAzVL9yxpsWWPv6Y2

Author: claude

tests/test_converter.py

@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+import subprocess
+import sys
 from unittest.mock import MagicMock, patch
 
 from openkb.converter import convert_document, get_pdf_page_count
@@ -164,6 +166,36 @@ def test_docx_conversion_enables_keep_data_uris(self, kb_dir, tmp_path):
         assert result.source_path.read_text(encoding="utf-8") == "converted markdown"
 
 
+# ---------------------------------------------------------------------------
+# Lazy markitdown import (guards the deferred-import optimization)
+# ---------------------------------------------------------------------------
+
+
+class TestLazyMarkItDownImport:
+    def test_importing_converter_does_not_load_markitdown(self):
+        """`import openkb.converter` must not pull in markitdown/magika/onnxruntime.
+
+        markitdown is imported lazily inside convert_document's Office-format
+        branch so `import openkb` stays cheap and a packaged build can drop
+        magika/onnxruntime. This runs in a fresh interpreter (a clean
+        sys.modules) so the assertion is deterministic regardless of what other
+        tests in this process already imported.
+        """
+        heavy = ("markitdown", "magika", "onnxruntime")
+        code = (
+            "import sys, openkb.converter; "
+            f"print(','.join(m for m in {heavy!r} if m in sys.modules))"
+        )
+        proc = subprocess.run(
+            [sys.executable, "-c", code],
+            capture_output=True,
+            text=True,
+        )
+        assert proc.returncode == 0, proc.stderr
+        loaded = proc.stdout.strip()
+        assert loaded == "", f"import openkb.converter eagerly loaded: {loaded}"
+
+
 # ---------------------------------------------------------------------------
 # _registry_path
 # ---------------------------------------------------------------------------