Introduce app context switch for setting MSF=true by default

[apoorvdeshmukh]

Description

This PR introduces new app context switch Switch.Microsoft.Data.SqlClient.EnableMultiSubnetFailoverByDefault to set MultiSubnetFailover=true by default in connection string.

Usage

In application

AppContext.SetSwitch("Switch.Microsoft.Data.SqlClient.EnableMultiSubnetFailoverByDefault", true);

Or In config file runtimeconfig.json

{
  "configProperties": {
    "Switch.Microsoft.Data.SqlClient.EnableMultiSubnetFailoverByDefault": true
  }
}

Or in App.Config

<runtime>
  <AppContextSwitchOverrides value="Switch.Microsoft.Data.SqlClient.EnableMultiSubnetFailoverByDefault=true" />
</runtime>

Issues

AB#40809

Testing

Enhanced LocalAppContextSwitchesTest.cs to test new app context switch.
Added unit tests in SqlConnectionStringTest.cs

Guidelines

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[Copilot]

Pull request overview

This PR introduces a new app context switch Switch.Microsoft.Data.SqlClient.EnableMSFByDefaultInConnString that allows applications to change the default value of the MultiSubnetFailover connection string property from false to true. This enables parallel IP connection attempts by default in multi-subnet environments, which can improve connection times for Always On availability groups.

Key changes:

• Implements the app context switch following established patterns in the codebase
• Changes DbConnectionStringDefaults.MultiSubnetFailover from a const to a property that reads the switch value
• Adds comprehensive unit tests covering various scenarios including explicit overrides and FailoverPartner validation
• Properly integrates with the existing test infrastructure for app context switch testing

Reviewed changes

Copilot reviewed 5 out of 5 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	Adds the new app context switch constant, static field, and public property with XML documentation
src/Microsoft.Data.SqlClient/src/Microsoft/Data/Common/ConnectionString/DbConnectionStringDefaults.cs	Changes MultiSubnetFailover from const to property that reads from the app context switch
src/Microsoft.Data.SqlClient/tests/Common/LocalAppContextSwitchesHelper.cs	Adds test helper infrastructure to manipulate and restore the switch state for testing
src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/SqlConnectionStringTest.cs	Adds comprehensive unit tests for the new switch behavior including edge cases
src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/LocalAppContextSwitchesTest.cs	Updates default value assertion test to include the new switch

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.