Ignore unmatched Creative Commons license end tags

[terminalchai]

An unmatched Creative Commons license closing tag, such as </creativeCommons:license>, currently makes feedparser.parse() raise KeyError: 'license'. Return when there is no matching license element, preserving existing license links and surrounding text without adding a spurious link.

This addresses the Creative Commons license cases reported in #599; other malformed-feed cases in that issue are outside this change.

Adds 14 regression cases covering bare closing tags, feed and entry contexts, namespace aliases, repeated closing tags after a valid license, and closing tags within title text. Includes a changelog fragment.

Validation on Windows with Python 3.11:

• All 14 new regression cases failed with KeyError before the fix and pass afterward.
• Full suite: 4,311 passed, 8 skipped.
• Black, isort, flake8, mypy (33 source files), and git diff --check pass.

Prepared with AI assistance; the diff and regression results were reviewed before submission.