Fix unclosed <details> tag when primer comment is truncated mid-block

When truncation cuts inside an open <details> block, the closing tag
was missing, causing GitHub to render the rest of the PR comment inside
the collapsed section. Now detect the open tag and close it before
appending the truncation notice. Add a dedicated test case for this.

Author: Pierre-Sassoulas

pylint/testutils/_primer/primer_compare_command.py

@@ -113,11 +113,19 @@ def _truncate_comment(self, comment: str) -> str:
                 f"*This comment was truncated because GitHub allows only"
                 f" {MAX_GITHUB_COMMENT_LENGTH} characters in a comment.*"
             )
+            # Reserve space for the suffix and a potential </details> closing tag.
+            suffix = f"\n{truncation_information}\n\n"
+            closing_tag = "</details>\n"
             max_len = (
                 MAX_GITHUB_COMMENT_LENGTH
                 - len(hash_information)
-                - len(truncation_information)
+                - len(suffix)
+                - len(closing_tag)
             )
-            comment = f"{comment[:max_len - 10]}...\n\n{truncation_information}\n\n"
+            comment = comment[: max_len - 10] + "...\n"
+            # Close any <details> tag left open by the truncation.
+            if comment.count("<details>") > comment.count("</details>"):
+                comment += closing_tag
+            comment += suffix
         comment += hash_information
         return comment

tests/testutils/_primer/cases/message_changed/expected_truncated.txt

@@ -1,19 +1,16 @@
 🤖 **Effect of this PR on checked open source code:** 🤖
 
 
-
 **Effect on [astroid](https://github.com/pylint-dev/astroid):**
+
 The following messages are now emitted:
 
 <details>
-
 1) locally-disabled:
 *Locally disabling redefined-builtin [we added some text in the message] (W0622)*
 https://github.com/pylint-dev/astroid/blob/123456789abcdef/astroid/__init__.py#L91
-
 </details>
-
-The followi...
+...
 
 *This comment was truncated because GitHub allows only 525 characters in a comment.*
 

tests/testutils/_primer/cases/message_changed/expected_truncated_in_details.txt

@@ -0,0 +1,15 @@
+🤖 **Effect of this PR on checked open source code:** 🤖
+
+
+**Effect on [astroid](https://github.com/pylint-dev/astroid):**
+
+The following messages are now emitted:
+
+<details>
+1) locally-disabled:
+*Locally disabling redefined-builtin [we added some text in the message...
+</details>
+
+*This comment was truncated because GitHub allows only 420 characters in a comment.*
+
+*This comment was generated for commit v2.14.2*

tests/testutils/_primer/test_primer.py

@@ -86,6 +86,19 @@ def test_truncated_compare(self) -> None:
             )
         assert len(content) < max_comment_length
 
+    def test_truncated_compare_in_details(self) -> None:
+        """Test for the truncation of comments that are too long inside details."""
+        max_comment_length = 420
+        directory = CASES_PATH / "message_changed"
+        with patch(
+            "pylint.testutils._primer.primer_compare_command.MAX_GITHUB_COMMENT_LENGTH",
+            max_comment_length,
+        ):
+            content = self.__assert_expected(
+                directory, expected_file=directory / "expected_truncated_in_details.txt"
+            )
+        assert len(content) < max_comment_length
+
     @staticmethod
     def __assert_expected(
         directory: Path,