fix(gateway): reject file paths in get_command() + add file-drop tests

Gateway's get_command() treated /path/to/file as command 'path' because
it had no / check after stripping the leading slash. The CLI already
solved this with _looks_like_slash_command() but the gateway adapter
layer was never patched.

Adds the same heuristic to MessageEvent.get_command(): valid command
names never contain /, file paths always do.

Also adds 28 regression tests for the CLI's _detect_file_drop().

Gateway fix from #6978 (@ygd58), tests from #6963 (@betamod).

gateway/platforms/base.py

@@ -613,6 +613,9 @@ class MessageEvent:
         raw = parts[0][1:].lower() if parts else None
         if raw and "@" in raw:
             raw = raw.split("@", 1)[0]
+        # Reject file paths: valid command names never contain /
+        if raw and "/" in raw:
+            return None
         return raw
     
     def get_command_args(self) -> str: