fix: prevent duplicate ECS agent provisioning while JNLP agent is starting, #311, #416

[mslipets]

ECS agents use JNLP (agent-initiated connection), so they are invisible to Jenkins' connectingExecutors metric while the container is starting up.
This creates a gap: once the PlannedNode future resolves (nearly instantly after ProvisioningCallback returns the ECSSlave), the node is removed from pendingLaunches but isn't yet online.
Every subsequent ECSProvisioningStrategy cycle - which fires every 10 seconds - sees excessWorkload > 0 and launches another duplicate ECS task.

The fix accounts for offline ECSSlave nodes matching the requested label as already-pending capacity, subtracting their executors from excessWorkload before the provisioning loop.
Failed agents are cleaned up by ECSLauncher.launch()'s catch block (agent.terminate()), so they don't linger as false pending capacity.

Fixes #311, #416

Testing done

5 automated tests added in ECSProvisioningStrategyTest covering:

Test	Scenario
noOfflineECSNodes_provisionsWhenWorkloadExists	Baseline: no pending nodes → provisions normally
offlineECSNodeMatchingLabel_suppressesProvisioning	Core fix: offline ECS node with matching label suppresses duplicate provision
offlineECSNodeDifferentLabel_provisionsNormally	Label filter: wrong-label offline node does not suppress provisioning
offlineECSNodeCoversPartOfWorkload_provisionsRemainder	Partial suppression: pending node covers part of queue, remainder is provisioned
multipleOfflineECSNodesCoversWorkload_suppressesProvisioning	Multiple pending nodes fully absorb workload

All 43 tests pass on Java 21 (BUILD SUCCESS, 3 pre-existing @Ignore skips in ECSServiceTest).

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[mslipets]

@Stericson Kindly requesting a review.