fix(python): bound excluded-element preflight

[jt55401]

Summary

• replace the Python excluded-element preflight regex with a deterministic quote-aware scanner
• reject incomplete excluded opening tags without regex backtracking
• add regressions for quoted delimiters and a distilled pathological input

Corpus reproducer

Common Crawl record d8851c6675546bf3 (125,610 bytes) now rejects in about 1.2 ms. The previous regex exceeded 12 seconds on the distilled reproducer and consumed a core for more than ten minutes in Study 1.

Verification

• 56 Python unit tests passed
• 118 shared conformance fixtures passed
• Python compilation passed
• git diff --check