🐛 manager: suppress expected leader-election-lost error on graceful shutdown

[liketosweep]

Fixes #3535

What the Issue Was

On graceful shutdown with LeaderElection enabled, cancelling the
leader election context triggers OnStoppedLeading, which pushes
errLeaderElectionLost onto errChan. The drain goroutine in
engageStopProcedure only filtered context.Canceled, so this
expected error was logged as unexpected:

level=ERROR msg="error received after stop sequence was engaged" err="leader election lost"

When combined with t.Output() in tests, this also causes panics or
data races as the manager writes to the test log after the test exits.

Solution

• Added a package-level errLeaderElectionLost sentinel for reliable
  errors.Is matching.
• Used the sentinel in OnStoppedLeading instead of an anonymous
  errors.New.
• Added the sentinel to the engageStopProcedure drain filter
  alongside context.Canceled.
  Genuine mid-run leader election loss (before stop is engaged) still
  propagates to Start's caller unchanged.

Changes

• pkg/manager/internal.go: sentinel + filter fix.
• pkg/manager/manager_test.go: regression test for graceful shutdown.

Testing

$ go test sigs.k8s.io/controller-runtime/pkg/manager -v \
  -run "should not log an error when leader election is lost during graceful shutdown"
--- PASS
PASS

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