Add dSTS integration tests

[XiaoxinMS2]

• Add 7 comprehensive integration tests for dSTS authority
• Tests cover certificate authentication, token caching, tenant switching, correlation ID, and error handling
• Store dSTS configuration securely in Key Vault (MISE-dSTS-CustomAppConfig)
• Remove hardcoded sensitive information for security
• All tests pass successfully (7/7)

Fixes #

Changes proposed in this request

Testing

Performance impact

Documentation

• All relevant documentation is updated.

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[Copilot]

Pull request overview

Adds new integration coverage for MSAL.NET’s dSTS (Distributed STS) authority by introducing a dedicated test suite that validates client-credentials scenarios and by wiring in a Key Vault secret name for retrieving dSTS app configuration.

Changes:

• Added a new DstsIntegrationTests headless integration test suite covering token acquisition, cache behavior, tenant override, correlation ID propagation, and error handling.
• Added a new Key Vault secret constant to retrieve dSTS app configuration from the lab Key Vault.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 10 comments.

File	Description
tests/Microsoft.Identity.Test.LabInfrastructure/KeyVaultSecrets.cs	Adds a new Key Vault secret name constant for retrieving dSTS app configuration JSON.
tests/Microsoft.Identity.Test.Integration.netcore/HeadlessTests/DstsIntegrationTests.cs	Introduces new dSTS headless integration tests for confidential client flows and related behaviors.

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