[6.0] Port #3841 Introduce app context switch for setting MSF=true by default

[apoorvdeshmukh]

Description

Ports #3841 to release/6.0

Issues

AB#41016

Guidelines

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[Copilot]

Pull request overview

This PR ports feature #3841 to the release/6.0 branch, introducing a new AppContext switch EnableMultiSubnetFailoverByDefault that allows users to configure the default value of the MultiSubnetFailover connection string property. The change enables applications to opt-in to using MultiSubnetFailover=true by default, which provides faster connection times in multi-subnet environments through parallel IP connection attempts.

• Added new AppContext switch EnableMultiSubnetFailoverByDefault with default value of false
• Changed MultiSubnetFailover default from compile-time constant to runtime-configurable value
• Added test coverage for the new switch

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
LocalAppContextSwitches.cs	Adds the new EnableMultiSubnetFailoverByDefault property following the established Tristate lazy initialization pattern
DbConnectionStringCommon.cs	Changes MultiSubnetFailover default from const bool to static readonly bool referencing the new switch
SqlConnectionString.cs	Updates MultiSubnetFailover default to static readonly bool for consistency with DbConnectionStringDefaults
LocalAppContextSwitchesTests.cs	Adds test case verifying the new switch defaults to false

The implementation follows the established patterns in the codebase for AppContext switches and maintains backwards compatibility by defaulting to false. The changes are consistent across all modified files and properly tested.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 62.75%. Comparing base (fd34d5e) to head (a00f7fb).
⚠️ Report is 1 commits behind head on release/6.0.

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

Additional details and impacted files

@@               Coverage Diff               @@
##           release/6.0    #3852      +/-   ##
===============================================
- Coverage        63.68%   62.75%   -0.94%     
===============================================
  Files              285      285              
  Lines            59152    59160       +8     
===============================================
- Hits             37673    37126     -547     
- Misses           21479    22034     +555     

Flag	Coverage Δ
addons	92.58% <ø> (ø)
netcore	68.08% <85.71%> (+0.15%)	⬆️
netfx	63.81% <85.71%> (-1.48%)	⬇️

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