[test] Cover the empty-iterable and plain-assert-mode branches

Adds two regression tests to close the patch-coverage gaps in
``callbinrepr`` reported by codecov on PR pytest-dev#14523:
* a plugin returning a truthy-but-empty iterator (``iter([])``) to
  exercise the second ``if not new_expl: continue`` after
  ``materialize_with_truncation``.
* a ``--assert=plain`` run to exercise the false branch of the
  ``assertmode == "rewrite"`` guard.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Pierre-Sassoulas

testing/test_assertion.py

@@ -2299,8 +2299,7 @@ def raise_exit(obj):
 def test_plugin_hook_returning_none_is_skipped(pytester: Pytester) -> None:
     """A ``pytest_assertrepr_compare`` impl returning ``None`` is skipped
     so the next impl (or the built-in) can produce the explanation.
-    Covers the ``if new_expl is None: continue`` branch in
-    ``callbinrepr``.
+    Covers the ``if not new_expl: continue`` branch in ``callbinrepr``.
     """
     pytester.makeconftest(
         """
@@ -2323,6 +2322,53 @@ def test_diff():
     )
 
 
+def test_plugin_hook_returning_empty_iterator_is_skipped(pytester: Pytester) -> None:
+    """A plugin returning a truthy but ultimately empty iterable is
+    skipped after materialisation. Covers the second
+    ``if not new_expl: continue`` branch in ``callbinrepr``.
+    """
+    pytester.makeconftest(
+        """
+        def pytest_assertrepr_compare(op, left, right):
+            # An iterator object is truthy, so it slips past the first
+            # falsy check; once materialised through truncation it is
+            # empty and the dispatcher must move on.
+            return iter([])
+        """
+    )
+    pytester.makepyfile(
+        """
+        def test_diff():
+            assert {1, 2} == {1, 3}
+        """
+    )
+    result = pytester.runpytest()
+    # The built-in set-comparison explanation still reaches the user.
+    result.stdout.fnmatch_lines(
+        ["*Extra items in the left set:*", "*Extra items in the right set:*"]
+    )
+
+
+def test_callbinrepr_plain_assert_mode(pytester: Pytester) -> None:
+    """In ``--assert=plain`` mode ``callbinrepr`` skips the ``%`` escape.
+    Covers the false branch of ``if item.config.getvalue("assertmode")
+    == "rewrite"``.
+    """
+    pytester.makepyfile(
+        """
+        def test_diff():
+            assert {1, 2} == {1, 3}
+        """
+    )
+    result = pytester.runpytest("--assert=plain")
+    # In plain mode the comparator still runs via ``callbinrepr`` (it
+    # is the rewrite escaping that's skipped), so the explanation is
+    # still produced.
+    result.stdout.fnmatch_lines(
+        ["*Extra items in the left set:*", "*Extra items in the right set:*"]
+    )
+
+
 def test_exception_before_first_yield_emits_summary_and_notice(monkeypatch) -> None:
     """When the comparator raises *before* any explanation line has been
     yielded, ``assertrepr_compare`` should still produce the summary so