Port #3841 to 6.1: Introduce app context switch for setting MSF=true

[apoorvdeshmukh]

Ports #3841 to release branch 6.1

Issues

AB#41015

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[Copilot]

Pull request overview

This PR ports #3841 to the release/6.1 branch, introducing an app context switch (Switch.Microsoft.Data.SqlClient.EnableMultiSubnetFailoverByDefault) that allows applications to enable MultiSubnetFailover by default without explicitly setting it in each connection string. When the switch is set to true, MultiSubnetFailover defaults to true instead of false, enabling parallel IP connection attempts for improved connection times in multi-subnet environments.

Key changes:

• Added new app context switch EnableMultiSubnetFailoverByDefault with tristate lazy initialization pattern
• Changed MultiSubnetFailover default from const to static property to allow runtime evaluation based on the app context switch
• Added comprehensive test coverage for the new switch behavior including unit and functional tests

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/LocalAppContextSwitches.cs	Added EnableMultiSubnetFailoverByDefault property and backing field with tristate lazy initialization
src/Microsoft.Data.SqlClient/src/Microsoft/Data/Common/ConnectionString/DbConnectionStringDefaults.cs	Changed MultiSubnetFailover from const to static property that evaluates the app context switch
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlConnectionString.cs	Changed DEFAULT.MultiSubnetFailover from const to static property to support runtime evaluation
src/Microsoft.Data.SqlClient/tests/Common/LocalAppContextSwitchesHelper.cs	Added test helper infrastructure for manipulating and restoring the new switch value
src/Microsoft.Data.SqlClient/tests/FunctionalTests/LocalAppContextSwitchesTests.cs	Added test to verify the new switch defaults to false
src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/SqlConnectionStringTest.cs	Added parameterized test for MultiSubnetFailover behavior with various combinations of connection string values and app context switch states; includes commented-out test for FailoverPartner validation

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 64.30%. Comparing base (8291391) to head (99d6fa3).
⚠️ Report is 1 commits behind head on release/6.1.

Files with missing lines	Patch %	Lines
...icrosoft/Data/SqlClient/LocalAppContextSwitches.cs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release/6.1    #3851      +/-   ##
===============================================
- Coverage        66.21%   64.30%   -1.91%     
===============================================
  Files              279      279              
  Lines            53293    53301       +8     
===============================================
- Hits             35286    34275    -1011     
- Misses           18007    19026    +1019     

Flag	Coverage Δ
addons	90.82% <ø> (ø)
netcore	68.44% <85.71%> (-4.31%)	⬇️
netfx	67.78% <85.71%> (+1.91%)	⬆️

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