Use rebase merge method in Mergify for linear history and DCO compliance (#2540)

## Summary
- Changes `merge_method` from `merge` to `rebase` to comply with the
repo's linear history branch protection setting
- Removes `commit_message_template` which was only relevant for squash
merges and would have caused DCO failures (hardcoded `Signed-off-by:
Mergify <noreply@mergify.com>` doesn't match the commit author)
- Rebase keeps original signed commits intact, satisfying both DCO and
linear history requirements

## Test plan
- [ ] Verify Mergify auto-queues and merges a PR as `mergify[bot]`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: dsikka

.github/mergify.yml

@@ -3,18 +3,15 @@ merge_queue:
 
 queue_rules:
   - name: default
-    merge_method: merge
+    merge_method: rebase
     batch_size: 1
-    commit_message_template: |
-      {{ title }} (#{{ number }})
-
-      {{ body }}
-
-      Signed-off-by: Mergify <noreply@mergify.com>
     queue_conditions:
       - check-success=DCO
       - check-success=quality-check
-      - check-success-or-neutral=transformers-tests
+      - or:
+        - check-success=transformers-tests
+        - check-neutral=transformers-tests
+        - check-skipped=transformers-tests
       - check-success=base-tests (3.10)
       - check-success=base-tests (3.13)
       - check-success=pytorch-tests (3.10)
@@ -23,7 +20,10 @@ queue_rules:
     merge_conditions:
       - check-success=DCO
       - check-success=quality-check
-      - check-success-or-neutral=transformers-tests
+      - or:
+        - check-success=transformers-tests
+        - check-neutral=transformers-tests
+        - check-skipped=transformers-tests
       - check-success=base-tests (3.10)
       - check-success=base-tests (3.13)
       - check-success=pytorch-tests (3.10)
@@ -38,7 +38,10 @@ pull_request_rules:
       - "#approved-reviews-by>=2"
       - check-success=DCO
       - check-success=quality-check
-      - check-success-or-neutral=transformers-tests
+      - or:
+        - check-success=transformers-tests
+        - check-neutral=transformers-tests
+        - check-skipped=transformers-tests
       - check-success=base-tests (3.10)
       - check-success=base-tests (3.13)
       - check-success=pytorch-tests (3.10)