Enable per-bookmark locking by default in tests

Summary:
Set `scm/mononoke:per_bookmark_locking` to `true` in the
canonical JustKnobs defaults file used by both integration tests and
Rust unit tests.

This means new pushrebase tests automatically run with per-bookmark
locking enabled, matching the production target state. Existing tests
that explicitly call `merge_just_knobs` to enable it are now redundant
but harmless. The shadow-mode test explicitly sets it to `false` and
continues to work correctly.

Reviewed By: lmvasquezg

Differential Revision: D101818448

fbshipit-source-id: 56f15f7c8d01eb50cfb64d69bc28686fff02156e

Author: RajivTS

eden/mononoke/common/mononoke_macros/just_knobs_defaults/just_knobs.json

@@ -72,7 +72,7 @@
     "scm/mononoke:cross_repo_pause_backsyncer": false,
     "scm/mononoke:allow_bare_author_unixname": true,
     "scm/mononoke:lock_users_directory": false,
-    "scm/mononoke:per_bookmark_locking": false,
+    "scm/mononoke:per_bookmark_locking": true,
     "scm/mononoke:per_bookmark_locking_shadow": false,
     "scm/mononoke:add_acl_manifest_pointer": true
   },

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

@@ -3344,6 +3344,7 @@ mod tests {
             "scm/mononoke:pushrebase_dry_run_merge_resolution".to_string() => KnobVal::Bool(false),
             "scm/mononoke:pushrebase_enable_merge_resolution".to_string() => KnobVal::Bool(false),
             "scm/mononoke:pushrebase_merge_resolution_derive_fsnodes".to_string() => KnobVal::Bool(true),
+            "scm/mononoke:per_bookmark_locking".to_string() => KnobVal::Bool(false),
         }));
     }