Fix mention resolution to include known authors from payload

Issue: resolveAllowedMentionsFromPayload was passing empty string to resolveMentionsLazily,
which meant no mentions were extracted from text, so known authors were never added to the
allowed list. This caused "@pelikhan" and other team members to not be recognized as allowed mentions.

Fix: Create fake text with @mentions of known authors so they get processed by the mention
resolution logic. This ensures known authors (PR author, issue author, assignees) are always
included in the allowed mentions list along with collaborators.

The mention resolution now correctly:
1. Extracts known authors from event payload (issue/PR author, assignees, etc.)
2. Creates mention text from known authors
3. Resolves mentions which checks against collaborators cache
4. Returns combined list of allowed mentions for output sanitization

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

Author: Copilot