Add Scuba logging to MergeResolutionLostOnRetry invariant

Summary:
Add Scuba logging to the fail-closed invariant that detects when merge
resolution is lost during CAS retries. The existing ODS counter
(`merge_resolution_lost_on_retry`) fires but carries no investigation
context.

The new Scuba log writes to `mononoke_land_service` with:
- `log_tag=MergeResolutionLostOnRetry` for filtering
- `repo_name` for identifying the affected repository
- `retry_num` for understanding the retry context
- A descriptive violation message

This enables oncall to quickly investigate invariant violations by
querying `mononoke_land_service` with the log_tag filter, rather than
relying solely on an ODS counter spike.

Note: A separate ODS Alarm should be configured on
`mononoke.pushrebase.*.merge_resolution_lost_on_retry` to page the
oncall when this invariant fires.

Reviewed By: YousefSalama

Differential Revision: D99841012

fbshipit-source-id: 74ed319c7e4fb65b6694fa476fd27019af46c20b

Author: RajivTS

eden/mononoke/features/pushrebase/src/lib.rs

@@ -910,6 +910,19 @@ async fn rebase_in_loop(
         // guards against future logic changes that might break that property.
         if any_attempt_resolved_conflicts && merge_resolved_paths.is_none() {
             STATS::merge_resolution_lost_on_retry.add_value(1, repo_args.clone());
+
+            // Log to Scuba for oncall visibility. The ODS counter alone
+            // doesn't carry enough context to investigate.
+            ctx.scuba().clone()
+                .add("log_tag", "MergeResolutionLostOnRetry")
+                .add("repo_name", repo.repo_identity().name())
+                .add("retry_num", retry_num.0 as i64)
+                .add(
+                    "merge_resolution_invariant_violation",
+                    "any_attempt_resolved_conflicts=true but final attempt has no merge_resolved_paths",
+                )
+                .log();
+
             return Err(PushrebaseInternalError::MergeResolutionLostOnRetry.into());
         }