Expose workflow lineage id on WorkflowMetadata

[pditommaso]

Summary

Expose the workflow's lineage identifier as a first-class property on WorkflowMetadata, populated by LinObserver at flow-create time. Other observers (notably the Tower observer) can now read session.workflowMetadata.lineageId and forward it to downstream systems without computing it themselves or making a secondary API round-trip to resolve it.

Concretely:

• WorkflowMetadata gains a public String lineageId field. Reflective toMap() picks it up automatically.
• The LID computation that used to live in LinObserver.onFlowBegin() moves to LinObserver.onFlowCreate(Session). Lifecycle in nextflow.Session: init() constructs observers, then workflowMetadata; start() then calls notifyFlowCreate() (fires onFlowCreate on every observer) before fireDataflowNetwork() calls notifyFlowBegin(). Computing the hash in onFlowCreate therefore (a) has session.workflowMetadata available, and (b) completes before any observer's onFlowBegin runs — regardless of plugin discovery order.
• The store save of the WorkflowRun value, the history log write, and the in-memory WorkflowOutput initialisation move with it.
• Value is the canonical lid://<hash> URI; null when the nf-lineage plugin is not active.

This is a building block for a Seqera Platform-side change (PLAT-5235) that will persist Workflow.lineageId on the workflow entity at trace-begin time, replacing the current secondary GET /lineage/resolve?sessionId=… lookup. That PR will land separately.

Test plan

• ./gradlew :nf-lineage:test — full module suite green, including an added assertion in LinObserverTest."should save workflow" that verifies setLineageId(_) is invoked on the metadata with a lid://-prefixed URI.
• ./gradlew :plugins:nf-tower:test — green (no source changes there; just verifying the moved lifecycle hook doesn't regress the consumer that reads workflowMetadata.toMap()).
• ./gradlew :nextflow:test --tests "*WorkflowMetadata*" — green.

Draft

Marked as draft because the matching Seqera Platform change (PR to follow, tracked under PLAT-5235) hasn't landed yet. The Nextflow side is forward-compatible on its own — older Platform versions just ignore the extra field — so this can merge independently once reviewed.

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