fix: two bugs in document extraction pipeline

Bug 1: _drain_pending did not call extract_documents on follow-up
messages arriving mid-turn. Documents attached to queued messages were
silently dropped because _build_user_content only handles images.
Fix: call extract_documents before _build_user_content in _drain_pending.

Bug 2: extract_documents read the entire file into memory (up to 50 MB)
just to check 16 bytes of magic header for MIME detection.
Fix: read only the first 16 bytes via open()+read(16) instead of
Path.read_bytes().

Added regression tests for both bugs.

Made-with: Cursor

nanobot/utils/document.py

@@ -251,8 +251,9 @@ def extract_documents(
             )
             continue
 
-        raw = p.read_bytes()
-        mime = detect_image_mime(raw) or mimetypes.guess_type(path_str)[0]
+        with open(p, "rb") as f:
+            header = f.read(16)
+        mime = detect_image_mime(header) or mimetypes.guess_type(path_str)[0]
         if mime and mime.startswith("image/"):
             image_paths.append(path_str)
         else: