Fix members autocomplete showing non-matches

What does this MR do?

This MR fixes members autocomplete showing non-matches.

Currently, member autocomplete using @ incorrectly shows non-matches. If you type @marge., you are still shown the complete list of members even though there are no matches. The expected behaviour is that the autocomplete list only shows matches.

Issue: #330511 (closed)

Screenshots (strongly suggested)

before	after

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #330511 (closed)