deploy: Fix disallowed str::len in user-agent test

The repo disallows `str::len` (clippy::disallowed-methods) to avoid
confusing byte length with character count. Rework the assertion to
use `strip_prefix`, which both checks the prefix and gives us the
remainder to assert is non-empty.

Assisted-by: opencode (Claude Opus 4.8)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

crates/lib/src/deploy.rs

@@ -1402,13 +1402,12 @@ mod tests {
         let prefix = config
             .user_agent_prefix
             .expect("user_agent_prefix should be set");
-        assert!(
-            prefix.starts_with("bootc/"),
-            "User agent should start with bootc/"
-        );
         // Verify the version is present (not just "bootc/")
+        let version = prefix
+            .strip_prefix("bootc/")
+            .expect("User agent should start with bootc/");
         assert!(
-            prefix.len() > "bootc/".len(),
+            !version.is_empty(),
             "Version should be present after bootc/"
         );
     }