[cherry-pick] promote exception to property, add test (#3471)

[Westin Musser (westin-m)]

cherry pick #3471 into dev/8x

[Copilot]

Pull request overview

Cherry-picks the fix from #3471 to address a blocking-path bug in ConfigurationManager<T> where metadata fetch failures could lose the original exception during the backoff window, preventing callers from classifying client (4xx) vs server (5xx) errors.

Changes:

• Promote the blocking-path fetch failure exception from a local variable to an instance field so it persists across calls within _syncAfter backoff.
• Clear the persisted failure on successful retrieval/update to avoid stale errors.
• Add a regression test ensuring the inner exception (and HttpDocumentRetriever.StatusCode in Exception.Data) is preserved during the backoff window.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
test/Microsoft.IdentityModel.Protocols.OpenIdConnect.Tests/ConfigurationManagerTests.cs	Adds a regression test verifying the blocking backoff path preserves the original inner exception/status code.
src/Microsoft.IdentityModel.Protocols/Configuration/ConfigurationManager.cs	Introduces an instance field to persist the most recent blocking-path fetch failure across calls.
src/Microsoft.IdentityModel.Protocols/Configuration/ConfigurationManager_Blocking.cs	Uses the persisted failure field (set on failure, cleared on success) when throwing during the backoff window.

[Mallika Chennupaty (mdchennu)]

looks good to me, the comments added in the test were really helpful.

[debchoudhury-id4s]

was there a blocker preventing backporting #3471 earlier? Or was this timing intentional? Asking to learn about any possible release improvement.

[Westin Musser (westin-m)]

was there a blocker preventing backporting #3471 earlier? Or was this timing intentional? Asking to learn about any possible release improvement.

The only blocker was lack of reviews