fix: wrap over-length line flagged by pylint line-too-long

Extract the context slice into a local so the error message f-string fits within
the 120-char limit. pylint (C0301) now owns line-length since ruff cedes E501.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: farhan

xblocks_contrib/annotatable/annotatable.py

@@ -302,5 +302,6 @@ def definition_to_xml(self, resource_fs):
             # re-raise
             lines = self.data.split("\n")
             line, offset = err.position  # lint-amnesty, pylint: disable=unpacking-non-sequence
-            msg = f"Unable to create xml for block {self.usage_key}. Context: '{lines[line - 1][offset - 40 : offset + 40]}'"
+            context = lines[line - 1][offset - 40 : offset + 40]
+            msg = f"Unable to create xml for block {self.usage_key}. Context: '{context}'"
             raise SerializationError(self.usage_key, msg) from err