test: cover the dashboard widgets, the stale-screen banner, and the admin bar

[josephfusco]

The four lowest-covered units in the plugin, none of which had a test reaching their render or enqueue paths.

Closes #315
Closes #316

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5

Used for: Assisting with coverage measurement and test authoring

[github-actions]

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Props joefusco.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.15%. Comparing base (9ebaf50) to head (c4b8617).

Additional details and impacted files

@@              Coverage Diff              @@
##               main     #317       +/-   ##
=============================================
+ Coverage     79.62%   97.15%   +17.53%     
  Complexity      190      190               
=============================================
  Files            14       14               
  Lines          2076     2076               
=============================================
+ Hits           1653     2017      +364     
+ Misses          423       59      -364     

Flag	Coverage Δ
multisite	97.15% <ø> (+17.53%)	⬆️
phpunit	96.05% <ø> (+17.58%)	⬆️

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