ci(fuzz): fix stale fuzz target (pop_redo returns Option, not Result)

With the cargo-fuzz install fixed, the fuzz build now reaches target compilation
and fails: fuzz_state_machine.rs called .is_ok() on state.pop_redo(), which
returns Option<Operation> (E0599). Changed to .is_some(). Verified locally:
`cargo check --manifest-path impl/rust-cli/fuzz/Cargo.toml` now compiles all
fuzz targets clean (was: E0599).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KfgJznd6jzSeDYsSXGAXkU

Author: claude

impl/rust-cli/fuzz/fuzz_targets/fuzz_state_machine.rs

@@ -95,7 +95,7 @@ fuzz_target!(|data: &[u8]| {
         .map(|e| e.count())
         .unwrap_or(0);
 
-    while state.pop_redo().is_ok() {
+    while state.pop_redo().is_some() {
         // Keep undoing
     }