lib/composefs: Add error check for unsupported --download-only flag

The previous commit (c325582) added --download-only support for the
OSTree backend but did not add an error check for the composefs backend
where this feature is not yet implemented.

This commit adds a proper error message at the start of upgrade_composefs()
to explicitly bail out when --download-only is used, matching the pattern
used for other unsupported composefs features like --mutate-in-place and
edit operations.

Without this check, the flag would be silently ignored, potentially
causing confusion for users expecting download-only behavior.

Error message: "--download-only is not yet supported for composefs backend"

Signed-off-by: Wei Shi <wshi@redhat.com>

Author: shi2wei3

crates/lib/src/bootc_composefs/update.rs

@@ -266,6 +266,11 @@ pub(crate) async fn upgrade_composefs(
     storage: &Storage,
     composefs: &BootedComposefs,
 ) -> Result<()> {
+    // Download-only mode is not yet supported for composefs backend
+    if opts.download_only {
+        anyhow::bail!("--download-only is not yet supported for composefs backend");
+    }
+
     let host = get_composefs_status(storage, composefs)
         .await
         .context("Getting composefs deployment status")?;