Support configurable subject token types in identity assertion grants

[zhenyu-02]

Summary

• add a configurable RFC 8693 subject token type that defaults to the existing ID-token behavior
• expose ID-token and SAML 2.0 subject token type identifiers
• pass the configured token type through the identity assertion exchange and update callback documentation
• cover default ID-token, explicit SAML 2.0, form payload, and invalid configuration cases

Testing

• IdentityAssertionGrantTests: 24 passed per target framework across net10/net9/net8 (72 total)
• ModelContextProtocol.Tests net10: 2,362 passed, 5 skipped
• ModelContextProtocol.Core build: net10/net9/net8/netstandard2.0, 0 warnings and 0 errors
• dotnet format --verify-no-changes
• git diff --check

Fixes #1618