Fix SemaphoreLock context.Context implementation

[GavinFrazar]

This updates SempahoreLock to embed its context.Context and use WithCancelCause for its cancellation func, so that it properly implements the context.Context interface.
Specifically, the Err() method of a context.Context must not return a nil error if its Done channel has closed.

Fixes a panic that occurs if context.WithCancel(lease) is called after the lease context is done.
See: https://github.com/gravitational/teleport/actions/runs/13255328689/job/37001102495?pr=52023

{"timestamp":"2025-02-11T03:41:14Z","level":"debug","caller":"retryutils/retry.go:193","message":"Waiting to retry operation again","wait":826308778,"error":"cannot acquire semaphore user_task/deb47f2c-cfef-5b4d-bd50-15ed83d48e2d (err-max-leases)"}
{"timestamp":"2025-02-11T03:41:14Z","level":"warning","caller":"discovery/status.go:729","message":"Failed to create discover eks user task","component":"discovery:service","integration":"my-integration","issue_type":"eks-cluster-unreachable","aws_account_id":"123456789012","aws_region":"us-west-2","error":"context canceled"}
panic: context: internal error: missing cancel error

goroutine 6143 [running]:
context.(*cancelCtx).cancel(0xc002523b80, 0x0, {0x0, 0x0}, {0x1213ba00, 0x17048650})
	/opt/go/src/context/context.go:538 +0x346
context.(*cancelCtx).propagateCancel(0xc002523b80, {0x122031b8, 0xc007504d80}, {0x12174528, 0xc002523b80})
	/opt/go/src/context/context.go:473 +0x2c5
context.withCancel(...)
	/opt/go/src/context/context.go:273
context.WithCancel({0x122031b8, 0xc007504d80})
	/opt/go/src/context/context.go:236 +0xb1
github.com/gravitational/teleport/lib/srv/discovery.(*Server).acquireSemaphoreForUserTask(0xc008072708, {0xc008697a70, 0x24})
	/__w/teleport/teleport/lib/srv/discovery/status.go:609 +0x44c
github.com/gravitational/teleport/lib/srv/discovery.(*Server).mergeUpsertDiscoverEKSTask(0xc008072708, {{0xc00780cee2, 0xe}, {0x11226b9b, 0x17}, {0x113092c9, 0xc}, {0x113092bf, 0x9}, 0x0}, ...)
	/__w/teleport/teleport/lib/srv/discovery/status.go:760 +0x285
github.com/gravitational/teleport/lib/srv/discovery.(*Server).upsertTasksForAWSEKSFailedEnrollments(0xc008072708)
	/__w/teleport/teleport/lib/srv/discovery/status.go:728 +0x2bd
github.com/gravitational/teleport/lib/srv/discovery.(*Server).enrollEKSClusters(0xc008072708, {0x113092bf, 0x9}, {0xc00780cee2, 0xe}, {0xc004e12d80, 0x24}, {0xc000c98240, 0x2, 0x2}, ...)
	/__w/teleport/teleport/lib/srv/discovery/kube_integration_watcher.go:318 +0x1ae6
created by github.com/gravitational/teleport/lib/srv/discovery.(*Server).startKubeIntegrationWatchers.func2 in goroutine 5320
	/__w/teleport/teleport/lib/srv/discovery/kube_integration_watcher.go:185 +0x154e

[public-teleport-github-review-bot]

@GavinFrazar See the table below for backport results.

Branch	Result
branch/v15	Failed
branch/v16	Failed
branch/v17	Create PR