Fix flaky test

[martincostello]

• Fix flaky test by just asserting the instance check does not throw - other tests already implicitly test whether an instance actually exists.
• Check that we don't accidentally try and delete the default instance.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.94%. Comparing base (a882bec) to head (232f9fa).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1401       +/-   ##
===========================================
+ Coverage   56.56%   90.94%   +34.37%     
===========================================
  Files          21       23        +2     
  Lines        1105     1137       +32     
  Branches       91       96        +5     
===========================================
+ Hits          625     1034      +409     
+ Misses        461       78      -383     
- Partials       19       25        +6     

Flag	Coverage Δ
linux	56.56% <ø> (+0.18%)	⬆️
macos	56.56% <ø> (ø)
windows	90.41% <ø> (?)

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[Copilot]

Pull request overview

This PR fixes a flaky test in the SQL LocalDB test suite and simplifies the fuzz testing validation logic. The main focus is on making tests more reliable by removing unnecessary checks and simplifying test assertions.

• Simplified the Can_Test_Whether_Instances_Exist() test to only verify the method doesn't throw, as other tests already validate the actual existence checking behavior
• Removed complex invalid character validation in fuzz tests, now only checking to avoid deletion of default instances ("v11.0" and "MSSQLLocalDB")

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/SqlLocalDb.Tests/SqlLocalDbApiTests.cs	Simplified the Can_Test_Whether_Instances_Exist() test by removing start/stop instance logic and changing to only assert that calling InstanceExists() doesn't throw
tests/SqlLocalDb.FuzzTests/FuzzTests.cs	Removed the InvalidNameChars HashSet and simplified SanitizeInstanceName() to only filter out default instance names instead of checking for invalid characters

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