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).