[truncate explanation] Fix edge case when we truncate due to max_chars

Previousely the added test case would output:
"...Full output truncated (0 lines hidden), use '-vv' to show"

Author: Pierre-Sassoulas

src/_pytest/assertion/truncate.py

@@ -101,16 +101,15 @@ def _truncate_explanation(
         # No truncation happened, so we do not need to add any explanations
         return truncated_explanation
 
-    truncated_line_count = len(input_lines) - len(truncated_explanation)
     if truncated_explanation[-1]:
         # Add ellipsis and take into account part-truncated final line
         truncated_explanation[-1] = truncated_explanation[-1] + "..."
-        if truncated_char:
-            # It's possible that we did not remove any char from this line
-            truncated_line_count += 1
     else:
         # Add proper ellipsis when we were able to fit a full line exactly
         truncated_explanation[-1] = "..."
+    truncated_line_count = (
+        len(input_lines) - len(truncated_explanation) + int(truncated_char)
+    )
     return [
         *truncated_explanation,
         "",

testing/test_assertion.py

@@ -1394,6 +1394,17 @@ def test_truncates_at_8_lines_when_there_is_one_line_to_remove(self) -> None:
         assert result == expl
         assert "truncated" not in result[-1]
 
+    def test_truncates_full_line_because_of_max_chars(self) -> None:
+        """A line is fully truncated because of the max_chars value."""
+        expl = ["a" * 10, "b" * 71]
+        result = truncate._truncate_explanation(expl, max_lines=10, max_chars=10)
+        assert result == [
+            "a" * 10,
+            "...",
+            "",
+            "...Full output truncated (1 line hidden), use '-vv' to show",
+        ]
+
     def test_truncates_edgecase_when_truncation_message_makes_the_result_longer_for_chars(
         self,
     ) -> None: