test(merge): scope ca_merge_ leftover check to scenario-local tmpdirs

The "no temporary merge files should remain on disk" step in
features/sandbox_merge_strategies.feature:36 scanned global
tempfile.gettempdir() for any ca_merge_* entry and asserted zero. Under
behave-parallel --processes 8 a sibling scenario's in-flight tmpdir in
the same shared /tmp could be observed mid-merge and trip the
assertion, even though GitMergeStrategy.merge cleans up its own tmpdir
in a finally block
(src/cleveragents/infrastructure/sandbox/merge.py:151-155).

Patch tempfile.mkdtemp inside cleveragents.infrastructure.sandbox.merge
for the @when step to track the tmpdirs this scenario's merge actually
creates, then assert those specific paths are gone in the @then step.
Production merge code is unchanged; the contract under test
(cleanup-on-success) is preserved; sibling scenarios sharing /tmp no
longer race the assertion.

ISSUES CLOSED: #7527

features/steps/sandbox_merge_strategies_steps.py

@@ -99,7 +99,22 @@ def step_merge_git(context: Context) -> None:
     """Perform a merge using the GitMergeStrategy."""
     _assert_git_available()
     strategy = GitMergeStrategy()
-    context.merge_result = strategy.merge(context.base, context.ours, context.theirs)
+    created_tmpdirs: list[str] = []
+    real_mkdtemp = tempfile.mkdtemp
+
+    def _tracked_mkdtemp(*args, **kwargs):
+        path = real_mkdtemp(*args, **kwargs)
+        created_tmpdirs.append(path)
+        return path
+
+    with patch(
+        "cleveragents.infrastructure.sandbox.merge.tempfile.mkdtemp",
+        side_effect=_tracked_mkdtemp,
+    ):
+        context.merge_result = strategy.merge(
+            context.base, context.ours, context.theirs
+        )
+    context.ca_merge_tmpdirs = created_tmpdirs
 
 
 @when("the changes are merged using the git strategy with git unavailable")
@@ -184,10 +199,20 @@ def step_merged_empty(context: Context) -> None:
 
 @then("no temporary merge files should remain on disk")
 def step_no_temp_files(context: Context) -> None:
-    """Verify that no ca_merge_ temp directories remain."""
-    tmp_root = tempfile.gettempdir()
-    leftover = [d for d in os.listdir(tmp_root) if d.startswith("ca_merge_")]
-    assert len(leftover) == 0, f"Found leftover temp directories: {leftover}"
+    """Verify the tmpdirs THIS scenario's merge created have been cleaned up.
+
+    Checking the specific paths the merge created (rather than scanning the
+    shared temp directory for any ``ca_merge_*`` entry) keeps the assertion
+    robust under ``behave-parallel`` — a sibling scenario's in-flight tmpdir
+    in the same shared ``/tmp`` is not our concern.
+    """
+    created = getattr(context, "ca_merge_tmpdirs", [])
+    assert created, (
+        "Expected the merge step to record at least one tracked tmpdir; "
+        "got none."
+    )
+    leftover = [d for d in created if os.path.exists(d)]
+    assert not leftover, f"Found leftover temp directories: {leftover}"
 
 
 @then("the merged content should be the incoming side")