Fix quadratic-time backtracking in attr_list attribute regex

[naaz234]

When the attr_list extension is enabled, AttrListTreeprocessor.BASE_RE matches the attribute content with ([^}\n ][^\n]*) and then allows trailing spaces before the closing brace with a separate [ ]*. [^\n]* already matches spaces, so both groups cover the same run of trailing spaces. When a block ends with an unclosed attribute list the closing brace is never found, and for a run of n spaces the engine retries the match from each of the n+1 ways to split the spaces between the two groups, which is O(n^2).

Markdown is routinely used to render untrusted input server-side, so this is reachable pre-auth as an algorithmic-complexity DoS (CWE-1333/CWE-407). A two-line block such as text {a followed by a long run of spaces hits BLOCK_RE, which is built from BASE_RE; HEADER_RE and INLINE_RE share the same shape.

Measured with markdown.markdown(text, extensions=['attr_list']) on 'text {a' + ' ' * n:

trailing spaces	before	after
25,000	300 ms	3 ms
50,000	1,213 ms	6 ms
100,000	3,565 ms	11 ms
200,000	10,680 ms	22 ms

The trailing [ ]* can only ever match zero characters in a successful match, because the greedy [^\n]* consumes the trailing spaces first and then gives back exactly enough for \} to match. Dropping it removes the ambiguity and makes the match linear. I verified the old and new patterns produce identical spans and capture groups across ~16k inputs (valid attribute lists, the existing test cases, and random brace/space/tab strings), so valid input is unaffected. Added a regression test and ran the full test suite and flake8.

This is the same class of defect fixed for ReferenceProcessor in #798.

[waylan]

Please review our AI Policy. Specifically, the following:

Contributors must be able to explain all changes upon request during review.

... The use of an AI agent to submit a pull request, issue, or comment autonomously is not permitted.

You description reads like it was written by AI. Also, as the PR template was deleted, I am assuming you had AI autonomously submit the PR. This presents 3 problems:

1. Because you didn't write the description yourself, we don't know if you are able to explain the changes.
2. Because the PR template was deleted, you did not disclose AI use.
3. Having AI submit the PR is expressly forbidden and while this is a simple change to the code which is relatively easy to review, the policy requires that we close the request.

That said, it is possible that this was not AI generated. If you can honestly confirm that, then we will proceed. Otherwise, this will be closed.

PS to other maintainers: this is going to be my standard response to AI submitted PRs/issues from now on. Feel free to respond in kind if you see a new PR first.