Fix AKV Provider ref project reference propagating ref assembly to downstream outputs

[Copilot]

When ReferenceType=Project, the AKV Provider referenced the MDS ref project (src/Microsoft.Data.SqlClient/ref/) without Private="false", causing the ref-only Microsoft.Data.SqlClient.dll to be copied into downstream outputs (e.g. ManualTests). This conflicts with the real implementation assembly at runtime.

Description

• Bug fix: Added Private="false" to the MDS ref ProjectReference in AzureKeyVaultProvider.csproj.

<ProjectReference Include="$(RepoRoot)/src/Microsoft.Data.SqlClient/ref/Microsoft.Data.SqlClient.csproj"
                  Condition="'$(ReferenceType)' != 'Package'"
                  Private="false" />

The ref assembly is still used for compilation (MDS types resolve correctly), but is no longer copied to the AKV Provider's output or propagated transitively to consuming projects. This matches the pattern used in the notsupported project.

Issues

Triggered by review comment on #4016.

Testing

No automated tests added — the fix prevents a build/runtime conflict that would manifest as a duplicate-assembly error in downstream outputs. The CI project-reference build path exercises this scenario.

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[Copilot]

Pull request overview

This PR fixes a build/runtime conflict where the AKV Provider's reference to the MDS ref project (src/Microsoft.Data.SqlClient/ref/) was inadvertently causing the ref-only Microsoft.Data.SqlClient.dll to be copied into downstream outputs (e.g., ManualTests). When a project simultaneously references both the real MDS implementation and the AKV Provider (which transitively brought the ref-only DLL along), two conflicting copies of Microsoft.Data.SqlClient.dll could end up in the output directory. The fix follows the same pattern already used in the notsupported project.

Changes:

• Added Private="false" to the MDS ref ProjectReference in the AKV Provider csproj, preventing the ref-only assembly from being copied to local output and propagated transitively to downstream consumers.

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