[7.0] Add type forwards to public Abstractions types

[paulmedynski]

Cherry-pick of #4067 to release/7.0

[Copilot]

Pull request overview

Cherry-picks #4067 into release/7.0 to preserve binary compatibility by type-forwarding several public authentication-related types from Microsoft.Data.SqlClient to the Microsoft.Data.SqlClient.Extensions.Abstractions assembly.

Changes:

• Add assembly-level TypeForwardedTo attributes for the moved Abstractions types.
• Wire the new type-forward source file into legacy netfx/netcore build and ref projects.
• Add a unit test validating the forwarded types still load via the Microsoft.Data.SqlClient assembly and originate from the Abstractions assembly.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/Microsoft.Data.SqlClient/tests/UnitTests/TypeForwardTests.cs	Adds coverage to ensure the forwarded Abstractions types remain loadable via Microsoft.Data.SqlClient and resolve to the Abstractions assembly.
src/Microsoft.Data.SqlClient/src/TypeForwards.Abstractions.cs	Introduces the actual type-forward declarations for the moved public types.
src/Microsoft.Data.SqlClient/netfx/src/Microsoft.Data.SqlClient.csproj	Includes the new type-forward source in the legacy netfx build project.
src/Microsoft.Data.SqlClient/netfx/ref/Microsoft.Data.SqlClient.csproj	Includes the new type-forward source in the legacy netfx reference-assembly project.
src/Microsoft.Data.SqlClient/netcore/src/Microsoft.Data.SqlClient.csproj	Includes the new type-forward source in the legacy netcore build project.
src/Microsoft.Data.SqlClient/netcore/ref/Microsoft.Data.SqlClient.csproj	Includes the new type-forward source in the legacy netcore reference-assembly project.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.66%. Comparing base (864f666) to head (93e3f2f).
⚠️ Report is 2 commits behind head on release/7.0.

❗ There is a different number of reports uploaded between BASE (864f666) and HEAD (93e3f2f). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (864f666)	HEAD (93e3f2f)
CI-SqlClient	1	0

Additional details and impacted files

@@               Coverage Diff               @@
##           release/7.0    #4117      +/-   ##
===============================================
- Coverage        73.07%   65.66%   -7.42%     
===============================================
  Files              280      275       -5     
  Lines            42997    65825   +22828     
===============================================
+ Hits             31422    43226   +11804     
- Misses           11575    22599   +11024     

Flag	Coverage Δ
CI-SqlClient	?
PR-SqlClient-Project	65.66% <ø> (?)

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