Fix prefix collisions in bare tool call names - #7667

A bare search_web{"query":"weather"} call is renamed to search when the registered tools are ['search', 'search_web']. This can select the wrong executor in chat or return the wrong function name through the API.

Capture the exact name already matched by the regex instead of selecting it again with startswith. Keep the nonempty-name guard so an empty tool list still produces no calls. JSON extraction and the returned content prefix are unchanged.

Fixes #7666

Tests

• python -m unittest discover -s tests -v: 10 standard-library tests passed on Python 3.12.15 and 3.13.16. Four regressions fail on unchanged source; six controls pass, including eleven other-format subcases.
• Real local tool loading and execution: unchanged source runs search; the fix runs search_web with the same arguments.
• Full modules compilation, repository-configured pycodestyle, additional ruff check, and git diff --check: passed.

No dependency changes. A full model/UI/API server was not run; the public parser and local tool loading/execution were exercised without mocking them.

Checklist:

• I have read the Contributing guidelines.