Fix Gitlab/JsonSafeParse: lib/gitlab/import/user_from_mention.rb

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Fix Gitlab/JsonSafeParse violations in this file by replacing Gitlab::Json.parse with Gitlab::Json.safe_parse.

Related MR: !215073 (merged)

Implementation Plan

A contributor needs to:

1. Replace Gitlab::Json.parse with Gitlab::Json.safe_parse in this file.

2. Verify with the MR reviewer that the new size and depth limits imposed by safe_parse do not break any existing functionalities. The safe_parse method includes built-in protections against:

   • Excessively large JSON payloads
   • Deeply nested JSON structures

   Ensure that legitimate use cases in this file are not affected by these limits.