fix(sealing): don't let a cancelled task block scheduling of other tasks

[hmdsefi]

Related Issues

Fixes #10262

Proposed Changes

• trySched drops queued requests whose context is done before the assigner runs. Schedule has already returned ctx.Err() to their callers, so there is nothing to respond to. The new RequestQueue.RemoveCancelled does this in one pass and keeps the order of the rest.
• LazyCtx.Val doesn't cache a failed call when the caller's context was cancelled, so the next caller calls Get again. Other errors are still cached, including a deadline from SelectorTimeout, so a slow worker still fails fast for the rest of the pass.
• Tests and a CHANGELOG entry.

Additional Info

The assigner builds one schedWorkerCache per pass, shared by all queued tasks, and LazyCtx cached the first TaskTypes result for each worker. Selector calls use a context derived from the task's context. When a cancelled task's call came first, context canceled was cached for that worker, and every other task in the pass failed with the same getting supported worker task types error as in the issue. The cancelled request could never be assigned, so it stayed in the queue and the next pass could hit it again.

Each change alone fixes the case in the issue. Dropping requests covers tasks cancelled before a pass, and the LazyCtx change covers a task that gets cancelled while a pass is running.

LazyCtx now uses a mutex instead of sync.Once. Concurrent callers still wait for the call in progress, as before. The scheduler's worker cache is its only user.

Tests:

• TestSchedCancelledRequestDoesNotBlockOthers runs one trySched pass with a cancelled request ahead of a live one, and a worker whose TaskTypes fails on a done context like an RPC client does. With the sched.go and getonce.go changes reverted, it logs trySched(1) req.Sel.Ok error: getting supported worker task types for both requests and neither is scheduled.
• TestRequestQueueRemoveCancelled and the new lib/lazy tests cover the two changes separately.
• go test ./storage/sealer/ ./lib/lazy/ passes, and golangci-lint reports no issues for both packages.

Checklist

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• Commits have a clear commit message.
• PR title conforms with contribution conventions
• Update CHANGELOG.md or signal that this change does not need it per contribution conventions
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green