fix: parse last XML answer in message completions

[zakazaka95]

Summary

• make XMLParser.parse_answer consistently select the last matching answer block for string and message completions
• add regression coverage for multiple answer tags within one assistant message

Fixes #2447

Testing

• pytest tests/test_xml_parser.py — 19 passed
• Ruff check and format check passed for both changed files

Note

Fix XMLParser.parse_answer to use last <answer> tag in message completions

In parse_answer, assistant message content is now parsed with last=True, so the last matching answer tag is returned instead of the first. Adds a test confirming 45 is extracted from a message with multiple <answer> tags.

^{Macroscope summarized fcd1cdd.}