Guard user search limit against malformed values

[Jorel97]

The authenticated user mention search endpoint parses limit with parseInt, but malformed values such as limit=abc produce NaN. The current clamp uses Math.max/Math.min directly on that value, so NaN can flow into supabase.limit(...) instead of falling back to the documented default of 10.

Expected behavior: malformed or non-finite limits should use the default limit, while valid values remain clamped to 1..20.