fix(proxy): catch markitdown missing-dependency errors with clean hint

When markitdown is installed but a format-specific sub-dependency is
missing (e.g. pdfminer-six for PDF), translate the raw traceback into
a DependencyMissingError with the correct install command.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

markitect/proxy/extractors/markitdown_ext.py

@@ -67,7 +67,20 @@ class MarkitdownExtractor(BaseExtractor):
             )
 
         md = MarkItDown()
-        result = md.convert(str(source_path))
+        try:
+            result = md.convert(str(source_path))
+        except Exception as exc:
+            # Catch markitdown's FileConversionException (and sub-type
+            # MissingDependencyException) and surface a clean install hint.
+            msg = str(exc)
+            if "MissingDependency" in type(exc).__name__ or "MissingDependency" in msg:
+                ext = source_path.suffix.lstrip(".")
+                raise DependencyMissingError(
+                    f"markitdown needs an extra dependency for {ext} files.",
+                    package=f"markitdown-no-magika[{ext}]",
+                    install_hint=f'pip install "markitdown-no-magika[{ext}]"',
+                ) from exc
+            raise
         return ExtractionResult(
             content=result.text_content,
             extractor=self.name,