Fix AppContext switch race in unit tests

[paulmedynski]

Description

Fixes a unit-test race where SqlConnectionOptionsTest temporarily enables the process-wide cached EnableMultiSubnetFailoverByDefault switch while simulated-server failover tests run concurrently.

• Adds non-parallel xUnit collections for tests that mutate cached AppContext switches and for simulated-server tests that include switch-mutating scenarios.
• Assigns all LocalAppContextSwitchesHelper users to the AppContext isolation collection.
• Explicitly sets MultiSubnetFailover = false in TransientFault_ShouldConnectToPrimary so the scenario does not inherit the process-wide default.
• No public API or product behavior changes.

Issues

Investigated from CI-SqlClient-Package run 21207.

Testing

• Full regular .NET 8 unit suite: 984 passed, 9 skipped, 0 failed.
• Original concurrent reproducer: passed 12 consecutive runs after failing 12/12 before the fix.
• Focused final run: 11 passed, 0 failed.

Checklist

• Tests added or updated
• Public API changes documented (not applicable; no public API changes)
• Verified against customer repro (not applicable; verified against CI failure)
• Ensure no breaking changes introduced

[Copilot]

Pull request overview

This pull request addresses an xUnit parallelism race in the UnitTests project where tests temporarily mutating cached, process-wide AppContext switch values could be observed by concurrently running simulated-server failover tests.

Changes:

• Introduces two non-parallel xUnit collections to isolate (1) AppContext switch mutation and (2) simulated-server tests that may trigger switch-mutation scenarios.
• Assigns LocalAppContextSwitchesHelper users to the AppContext isolation collection to prevent cross-test interference.
• Makes TransientFault_ShouldConnectToPrimary explicitly opt out of inheriting a process-wide default by setting MultiSubnetFailover = false.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/Microsoft.Data.SqlClient/tests/UnitTests/TestIsolationCollections.cs	Adds new xUnit collection definitions to disable parallelization for AppContext-switch tests and simulated-server tests.
src/Microsoft.Data.SqlClient/tests/UnitTests/SimulatedServerTests/ConnectionFailoverTests.cs	Moves the simulated-server suite onto the new non-parallel collection and pins MultiSubnetFailover=false for a specific scenario.
src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/SqlConnectionOptionsTest.cs	Assigns test class to AppContext switch isolation collection.
src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/SqlConnectionInternalTimeoutTests.cs	Assigns test class to AppContext switch isolation collection.
src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/LocalAppContextSwitchesTest.cs	Assigns test class to AppContext switch isolation collection.
src/Microsoft.Data.SqlClient/tests/UnitTests/ConnectionPool/WaitHandleDbConnectionPoolIdleTimeoutTest.cs	Assigns test class to AppContext switch isolation collection.
src/Microsoft.Data.SqlClient/tests/UnitTests/ConnectionPool/WaitHandleDbConnectionPoolBudgetTest.cs	Assigns test class to AppContext switch isolation collection.
src/Microsoft.Data.SqlClient/tests/UnitTests/ConnectionPool/ChannelDbConnectionPoolWarmupTest.cs	Assigns test class to AppContext switch isolation collection.
src/Microsoft.Data.SqlClient/tests/UnitTests/ConnectionPool/ChannelDbConnectionPoolTest.cs	Assigns test class to AppContext switch isolation collection.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.81%. Comparing base (25dba6b) to head (12638f3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4495      +/-   ##
==========================================
- Coverage   64.67%   62.81%   -1.87%     
==========================================
  Files         288      283       -5     
  Lines       44046    66979   +22933     
==========================================
+ Hits        28488    42072   +13584     
- Misses      15558    24907    +9349     

Flag	Coverage Δ
CI-SqlClient	?
PR-SqlClient-Project	62.81% <ø> (?)

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