Populate RetryAfter on SlidingWindowRateLimiter

[nwoolls]

SlidingWindowLease.FailedLease was a static singleton constructed once with retryAfter: null, so TryGetMetadata(RetryAfter) always returned false on a rejected lease regardless of limiter state. This mirrors the fix already in place for FixedWindowRateLimiter.

Unlike the fixed-window case, a sliding window frees permits one segment at a time rather than resetting all at once, so the estimate walks forward through _requestsPerSegment from the next segment to free until enough permits are accounted for, rather than assuming the very next replenishment tick is sufficient.

CreateFailedSlidingWindowLease reads _requestsPerSegment and _currentSegmentIndex, both of which are otherwise only mutated under the limiter's lock (in ReplenishInternal). Callers on the permitCount == 0 fast path previously read state outside the lock; that path now acquires the lock before computing the failed lease to avoid a torn read.

Fix #131175

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[Copilot]

Pull request overview

This PR fixes SlidingWindowRateLimiter failed leases always reporting no RetryAfter metadata by computing an estimated retry delay based on the sliding window’s per-segment state, aligning behavior with other rate limiter implementations.

Changes:

• Replace the static singleton failed lease path with per-failure SlidingWindowLease instances that include an estimated RetryAfter.
• Add CreateFailedSlidingWindowLease (lock-required) to compute retry estimates by walking _requestsPerSegment across segments.
• Expand tests to validate RetryAfter metadata behavior across common and edge-case scenarios (queued items, partial segment elapsed, window expired).

Reviewed changes

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

File	Description
src/libraries/System.Threading.RateLimiting/src/System/Threading/RateLimiting/SlidingWindowRateLimiter.cs	Compute and return failed leases with populated RetryAfter metadata, with locking to avoid torn reads.
src/libraries/System.Threading.RateLimiting/tests/SlidingWindowRateLimiterTests.cs	Add/adjust tests to validate RetryAfter metadata for failed leases and related timing scenarios.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

[nwoolls]

Not sure on the failing builds — I tried running them locally (if I read the YAML correctly) and they succeeded for me on macOS.

./build.sh -s clr+clr.runtime+libs+packs+libs.tests -c Release /p:ArchiveTests=true /p:DevTeamProvisioning=- /p:UseMonoRuntime=false /p:UseNativeAOTRuntime=false /p:RunSmokeTestsOnly=true /p:BuildTestsOnHelix=true /p:EnableAdditionalTimezoneChecks=true /p:UsePortableRuntimePack=false /p:EnableAggressiveTrimming=true
...
Build succeeded.
    0 Warning(s)
    0 Error(s)

Time Elapsed 00:02:15.23