Fix ResourcesAggregator deadlock with virtual thread executor

[pditommaso]

Summary

Fix a deadlock in ResourcesAggregator.computeSummaryMap() that causes Nextflow to hang indefinitely during shutdown when generating the execution report.

Problem

ResourcesAggregator used the shared session ExecutorService — a ThreadPerTaskExecutor backed by virtual threads — to parallelize summary stats computation via invokeAll().

When other virtual threads (e.g. PublishDir file transfers) saturate or pin all ForkJoinPool carrier threads, the new tasks submitted by invokeAll can never be scheduled. Since invokeAll blocks until all futures complete, this creates a starvation deadlock: the main thread waits forever at computeSummaryMap() during onFlowComplete.

Observed in production — thread dump shows the main thread stuck at:

at java.util.concurrent.ThreadPerTaskExecutor.invokeAll(ThreadPerTaskExecutor.java:365)
at nextflow.trace.ResourcesAggregator.computeSummaryMap(ResourcesAggregator.groovy:75)
at nextflow.trace.ResourcesAggregator.computeSummaryList(ResourcesAggregator.groovy:90)
at nextflow.trace.ResourcesAggregator.renderSummaryJson(ResourcesAggregator.groovy:105)
at nextflow.trace.ReportObserver.renderSummaryJson(ReportObserver.groovy:201)
...
at nextflow.Session.shutdown0(Session.groovy:777)

This is related to but independent of #6833 — that PR fixes one cause of carrier thread saturation (S3 delete), but ResourcesAggregator is vulnerable to any scenario where carrier threads are busy.

Fix

Replace the shared session virtual thread executor with a dedicated FixedThreadPool scoped to the computeSummaryMap() call. This is CPU-bound computation that:

• Benefits from platform threads (no I/O waiting)
• Must not compete with virtual threads for carrier thread scheduling
• Only runs once during shutdown, so the pool creation overhead is negligible

This also removes the Session dependency from ResourcesAggregator, simplifying both the class and its tests.

Test plan

• Existing ResourcesAggregatorTest tests pass (summary computation, JSON rendering, insertion order)
• ReportObserverTest passes
• nf-tower plugin compiles cleanly

🤖 Generated with Claude Code

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[bentsherman]

The analysis is slightly off -- PublishDir doesn't use the main session execService to publish files -- but I think the change is still reasonable

[pditommaso]

I think it get confused with the similar one #6833, but yes, it's correct