Implement Oplock Break Acknowledgement handler (MS-SMB2 §2.2.24)

- Parse client's OPLOCK_BREAK_ACK
- Update Open.oplock_level in Open struct
- Update OplockManager entry via update_oplock_level()
- Return confirmation response

All 229 tests pass.

vendor/smb-server/src/oplock.rs

@@ -191,6 +191,21 @@ pub struct LockRange {
     pub tree_id: u32,
 }
 
+impl OplockManager {
+    /// Update oplock level after client acknowledges a break (MS-SMB2 §2.2.24).
+    pub async fn update_oplock_level(&self, path: &SmbPath, file_id: FileId, new_level: u8) {
+        let mut file_opens = self.file_opens.write().await;
+        if let Some(entries) = file_opens.get_mut(path) {
+            for entry in entries.iter_mut() {
+                if entry.file_id == file_id {
+                    entry.oplock_level = new_level;
+                    break;
+                }
+            }
+        }
+    }
+}
+
 /// Global byte-range lock manager (MS-SMB2 §3.3.1.9).
 pub struct LockManager {
     /// FileId → active locks on that file.