Implement warning log for conflicting Authority and Instance/TenantId/B2C configs (no exception)

[jmprieur]

Background

When both Authority and Instance/TenantId are provided, the current behavior is to silently ignore Authority. To avoid breaking changes, log a warning rather than throwing an exception. Ensure clear diagnostics for both AAD B2C (including policies, user-flows, /tfp/ etc.), and CIAM authorities

Implementation

• In MergedOptions.ParseAuthorityIfNecessary and/or merging points, emit a warning via ILogger when both Authority and Instance or TenantId are set.
• Enhance existing merge logic for B2C authorities so warnings clarify which configuration is being honored.
• Add test case using your test harness to verify warning is emitted.
• Use high performance logging.

Example warning format

_logger?.LogWarning("Both Authority and Instance/TenantId are set in MicrosoftIdentityApplicationOptions. Authority will be ignored. To avoid confusion, use one or the other.");

Sample conflicting config

option.Authority = "https://login.microsoftonline.com/tenantA/v2.0";
option.Instance = "https://login.microsoftonline.us/";
option.TenantId = "tenantB";
// B2C variant:
option.Authority = "https://contoso.b2clogin.com/tfp/contoso.onmicrosoft.com/B2C_1_signup_signin/v2.0";
option.Instance = "https://login.microsoftonline.com/";
option.TenantId = "contoso.onmicrosoft.com";
option.DefaultUserFlow = "B2C_1_signup_signin";

Acceptance Criteria

• Existing behavior preserved (no throw)
• New warning is logged in all affected code paths
• Unit tests verify warning is emitted
• B2C-specific paths are covered
• AAD-specific paths are covered
• CIAM-specific paths are covered
• Documentation updated: clearly explains precedence and warning

Related files

• MergedOptions.cs: https://github.com/AzureAD/microsoft-identity-web/blob/main/src/Microsoft.Identity.Web.TokenAcquisition/MergedOptions.cs
• TokenAcquisition.cs: https://github.com/AzureAD/microsoft-identity-web/blob/main/src/Microsoft.Identity.Web.TokenAcquisition/TokenAcquisition.cs
• TokenAcquirerTests/TokenAcquirer.cs

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Bridge: Propose exact logging diff(s) and add test mapping when Jean-Marc replies "Go ahead".