Hide generate commit message button when assistant is disabled (#26519)

Release Notes:

- Git Beta: Fixed the generate commit message button still showing when
the assistant is disabled.

crates/git_ui/Cargo.toml

@@ -19,6 +19,7 @@ test-support = ["multi_buffer/test-support"]
 [dependencies]
 anyhow.workspace = true
 askpass.workspace = true
+assistant_settings.workspace = true
 buffer_diff.workspace = true
 collections.workspace = true
 component.workspace = true