POC: Synchronous result-based validation fast path for issuer (#3459)

[debchoudhury-id4s]

Overview

Adds a synchronous validation path to the experimental model, scoped to the issuer check (#3459). When the configuration is already cached (or no ConfigurationManager is set), validation runs without the per-call Task and configuration awaits. The shipped TokenValidationParameters path and the async path are unchanged.

The goal is to keep this as a POC and capture benchmarking results comparing the synchronous and asynchronous paths.

Changes

• BaseConfigurationManager.TryGetCurrentConfiguration(out BaseConfiguration?) — synchronous config peek; base returns false, ConfigurationManager<T> overrides it and reuses the async fast-path predicate so the two can't drift.
• ISynchronousIssuerValidator + Validators.ValidateIssuer/ValidateIssuerInternal — synchronous issuer validation on the peeked config, implemented by DefaultIssuerValidator.
• JsonWebTokenHandler.ValidateToken(...) + private ValidateJWS/ValidateJWE — synchronous pipeline; runs sync on a cache hit, otherwise falls back to ValidateTokenAsync. Records last-known-good on success.
• PublicAPI.Unshipped.txt, CHANGELOG.md, and ValidateTokenSyncTests (sync vs async golden master, including a cache-hit case).

Validation

Build clean (0 warnings, 0 errors) on all target frameworks. Microsoft.IdentityModel.Tokens.Tests, Microsoft.IdentityModel.JsonWebTokens.Tests, and Microsoft.IdentityModel.Protocols.Tests pass.

[Toidi357]

I think this is related to my intern project. I'm also working on making a synchronous result-based validation method ValidateToken, but doesn't have any asynchronous fallback. Async fallback creates sync-over-async (when you do .GetAwaiter().GetResult() issues which I'm hoping to avoid by adding methods to HttpDocumentRetriever, ConfigurationManager, and other related classes

[debchoudhury-id4s]

perf results are in Perf Comparison synchronous validation POC.docx.

TL;DR the POC is safe to be merged (not saying we should NOW). Check section 2 which does a sync vs async comparison showing how the POC reduces latency and improves perf