Resilience improvements for instance discovery

[bgavrilMS]

Fixes #5804 #5805

If instance discovery fails due to 404 or 502, it should not be attempted again
Instance discovery should have a reasonble timeout

[Copilot]

Pull request overview

Improves AAD instance discovery resilience and performance by avoiding repeated network instance discovery attempts when the endpoint is failing/unreachable, and by bounding discovery latency with a dedicated timeout.

Changes:

• Cache a fallback instance discovery entry when network instance discovery fails (non-invalid_instance) to avoid retrying discovery on subsequent token requests.
• Add a per-instance-discovery timeout (default 10s) by linking a timeout CancellationToken into the discovery request flow.
• Add unit tests covering caching-on-failure and timeout fallback behavior; add a rules doc for cross-SDK reference.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/Microsoft.Identity.Test.Unit/PublicApiTests/InstanceDiscoveryTests.cs	Adds tests ensuring instance discovery failures/timeouts are cached and not retried.
src/client/Microsoft.Identity.Client/Internal/RequestContext.cs	Makes UserCancellationToken settable to support temporary override during instance discovery.
src/client/Microsoft.Identity.Client/Instance/Discovery/NetworkMetadataProvider.cs	Adds instance discovery timeout and links it into the outgoing request.
src/client/Microsoft.Identity.Client/Instance/Discovery/InstanceDiscoveryManager.cs	Caches fallback metadata on discovery failure; updates warning text.
docs/instance-discovery-rules.md	Adds a detailed description of instance discovery behavior and error-handling rules.

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

[gladjohn]

Can we add one regression test for caller-driven cancellation here? The new timeout path temporarily swaps RequestContext.UserCancellationToken to a linked CTS and relies on the outer catch filter to distinguish timeout from user cancellation. The current timeout test throws TaskCanceledException directly from the mock handler, so it doesn’t exercise the linked-token path or verify that a caller cancellation still bubbles instead of falling back.

[gladjohn]

looks good, one follow-up test needed.