Add unit tests with HTTP mocking for vanilla dSTS + fix Authority par…

[XiaoxinMS2]

…sing

Adds 6 HTTP-mocked unit tests for the vanilla dSTS (Dedicated Security Token Service) token-acquisition path in Microsoft.Identity.Web, and fixes MergedOptions.ParseAuthorityIfNecessary so that the natural / documented dSTS configuration form works end-to-end:

options.Authority = "https://{host}/dstsv2/{tenantGuid}";

Without the fix, the AAD-style parser took the literal "dstsv2" as the tenant and dropped the actual tenant GUID, producing an authority MSAL rejected with "The DSTS authority URI should have at least 2 segments...".

Tests cover: token endpoint URI; client_credentials grant body; second-call cache hit; OAuth2 error -> MsalServiceException mapping; SendX5C=true includes x5c JWT header; SendX5C=false omits it. All tests use the existing MockHttpClientFactory infrastructure (no real network / Key Vault / cert) and run in any CI environment.

No public API changes.

Add unit tests with HTTP mocking for vanilla dSTS scenarios + fix Authority-only configuration

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Summary of the changes (Less than 80 chars)

Description

Adds 6 HTTP-mocked unit tests covering the vanilla dSTS (Dedicated Security Token Service) token-acquisition path in Microsoft.Identity.Web, and fixes a parser bug in MergedOptions.ParseAuthorityIfNecessary that prevented the natural / documented dSTS configuration form (options.Authority = "https://{host}/dstsv2/{tenantGuid}") from working.

Both changes ship together because the new tests use the natural configuration form, which is the form the parser fix unblocks.

Vanilla dSTS support in Id.Web previously had zero unit-test coverage — the only assertion lived in integration tests that required a real dSTS deployment + Key Vault certificate, so they couldn't run in CI. Anyone refactoring MergedOptions / TokenAcquirerFactory could silently break dSTS token acquisition without any signal.

Fixes #{bug number} (in this specific format)

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[cpp11nullptr]

nit: once it's merged, it'd be not obvious which PR is meant there (without using git blame), perhaps, we remove avoid this part - cf. PR review.