Add StoreWithSubjectName credential source

[iNinja]

Add StoreWithSubjectName credential source

Summary

Adds a new CredentialSource enum value, StoreWithSubjectName = 13, that allows a certificate to be
located in the Windows certificate store by matching a substring of its Subject field, rather than
requiring an exact distinguished name.

Motivation

The existing StoreWithDistinguishedName source requires the full X.500 distinguished name
(e.g. CN=MyCert, OU=..., O=..., C=...). In practice, callers often only know a short friendly name
fragment. StoreWithSubjectName fills this gap by accepting a partial, case-insensitive substring.

Changes

Area	Detail
CredentialSource.cs	New StoreWithSubjectName = 13 enum member with full XML doc and selection-semantics remarks
CredentialDescription.cs	New CertificateSubjectName property (backing field, _cachedId invalidation, copy constructor, Id switch case, CredentialType switch arm)
CredentialDescriptionJsonConverter.cs	Read and Write cases for StoreWithSubjectName / CertificateSubjectName
PublicAPI.Unshipped.txt (×6)	CertificateSubjectName get/set and CredentialSource.StoreWithSubjectName = 13 declared for all TFMs
CredentialDescriptionTest.cs	CertificateFromStoreBySubjectName test with subjectname_json / subjectname_csharp doc-snippet markers
CredentialDescriptionJsonConverterTest.cs	SerializeDeserialize_CertificateFromStoreBySubjectName round-trip test
CredentialDescriptionIdTest.cs	StoreWithSubjectName entry in the ID display dictionary; CachedId_InvalidatedWhen_CertificateSubjectName_Changes test
README.md	Mermaid class diagram updated with new property and enum value
docs/credentialdescription.md	Table row, JSON example, C# example, and certificate-store-paths note updated

Test results

All tests pass across all targeted frameworks:

Framework	Passed
net8.0	95
net10.0	95
net462	84

JSON converter tests are excluded on net462 (#if NET8_0_OR_GREATER), which accounts for the
difference in count.

[Copilot]

Pull request overview

Adds a new credential source option that allows describing Windows certificate-store credentials by (partial) certificate Subject name, extending the existing store-based lookup options in Microsoft.Identity.Abstractions.

Changes:

• Introduces CredentialSource.StoreWithSubjectName = 13 and a new CredentialDescription.CertificateSubjectName property.
• Extends CredentialDescriptionJsonConverter and ID/type derivation logic to support the new source.
• Updates tests and documentation (README + docs/credentialdescription.md) with examples and diagrams.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test/Microsoft.Identity.Abstractions.Tests/CredentialDescriptionTest.cs	Adds doc-snippet-backed test/example for store lookup by subject name
test/Microsoft.Identity.Abstractions.Tests/CredentialDescriptionJsonConverterTest.cs	Adds JSON round-trip test for StoreWithSubjectName
test/Microsoft.Identity.Abstractions.Tests/CredentialDescriptionIdTest.cs	Extends ID display coverage and cache invalidation test for the new property
src/Microsoft.Identity.Abstractions/PublicAPI/*/PublicAPI.Unshipped.txt	Declares new public API surface across TFMs
src/Microsoft.Identity.Abstractions/ApplicationOptions/CredentialSource.cs	Adds the new enum value and XML doc examples
src/Microsoft.Identity.Abstractions/ApplicationOptions/CredentialDescriptionJsonConverter.cs	Adds read/write support for CertificateSubjectName / StoreWithSubjectName
src/Microsoft.Identity.Abstractions/ApplicationOptions/CredentialDescription.cs	Adds property, copy support, ID generation, and credential type mapping
docs/credentialdescription.md	Adds documentation/examples for subject-name-based store lookup
README.md	Updates Mermaid diagram to include the new property/enum value

Comments suppressed due to low confidence (1)

src/Microsoft.Identity.Abstractions/PublicAPI/netstandard2.1/PublicAPI.Unshipped.txt:8

• PublicAPI.Unshipped.txt for netstandard2.1 ends with multiple blank lines. Please trim to match the formatting used in the other TFMs (typically a single trailing newline).