Fleet UI: Persist toast messages after a modal closes

[RachelElysia]

Issue

Cerra #17624

Description

• Persist flash message even when a modal closes

Note: I don't know why this recently broke, maybe the update to React, but the hide flash shouldn't have been in the onExit a modal call anywho.

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• Changes file added for user-visible changes in changes/ or orbit/changes/.
• Manual QA for all new/changed functionality

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.68%. Comparing base (57d00f3) to head (9ca3fef).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17674      +/-   ##
==========================================
- Coverage   65.68%   65.68%   -0.01%     
==========================================
  Files        1193     1193              
  Lines      108028   108026       -2     
  Branches     2574     2574              
==========================================
- Hits        70961    70959       -2     
  Misses      31701    31701              
  Partials     5366     5366              

Flag	Coverage Δ
frontend	51.94% <ø> (-0.02%)	⬇️

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