[6.0] Backport BAG failover fix. Ignore server provided failover partner.

[mdaigle]

Description

Backports #3625

[Copilot]

Pull Request Overview

This PR backports a fix for Basic Availability Group (BAG) failover behavior by introducing an AppContext switch that allows applications to ignore server-provided failover partners. This enables applications to maintain explicit control over failover configuration (e.g., using custom ports) instead of automatically accepting server-suggested partners.

Key changes include:

• Added a new IgnoreServerProvidedFailoverPartner AppContext switch with documentation
• Refactored ServerProvidedFailoverPartner from a private field with getter to an auto-property
• Updated failover logic to respect the new switch when determining actual failover partners

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 IgnoreServerProvidedFailoverPartner AppContext switch with property accessor and fixes a typo in existing documentation
src/Microsoft.Data.SqlClient/netfx/src/Microsoft/Data/SqlClient/SqlInternalConnectionTds.cs	Refactors failover partner storage, implements switch logic to conditionally use server-provided or connection-string failover partners
src/Microsoft.Data.SqlClient/netfx/src/Microsoft/Data/SqlClient/SqlCommand.cs	Updates property reference to use the new naming convention
src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/SqlInternalConnectionTds.cs	Mirrors the netfx implementation for .NET Core, applying the same refactoring and switch logic
src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/SqlCommand.cs	Updates property reference to use the new naming convention

[codecov]

Codecov Report

❌ Patch coverage is 72.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.69%. Comparing base (1542afd) to head (cccd71c).
⚠️ Report is 1 commits behind head on release/6.0.

Files with missing lines	Patch %	Lines
...crosoft/Data/SqlClient/SqlInternalConnectionTds.cs	68.42%	6 Missing ⚠️
...icrosoft/Data/SqlClient/LocalAppContextSwitches.cs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release/6.0    #3703      +/-   ##
===============================================
+ Coverage        75.51%   75.69%   +0.17%     
===============================================
  Files              244      244              
  Lines            40212    40221       +9     
===============================================
+ Hits             30368    30445      +77     
+ Misses            9844     9776      -68     

Flag	Coverage Δ
addons	92.58% <ø> (ø)
netcore	75.56% <72.00%> (+0.17%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[benrr101]

I don't really feel strongly enough about "" vs string.Empty to reject. But I'd happily re-approve if you did want to take that change.