feat(workflows): save BPMN documents with subprocesses

[sfmskywalker]

Closes #1032. Part of elsa-workflows/elsa-core#7909.

Why

#1031 refused to save a BPMN document that declared a subprocess, transaction or event subprocess. The core document PUT used to write those bodies empty. elsa-workflows/elsa-core#8077 fixed that: the PUT now restores each kept subprocess's stored body, with every binding inside it, by element id. The contract is unchanged.

What changed

• Removed the save refusal: BpmnDocumentFailureReason.SubProcessContentNotCarried, the SubProcessIds check in BpmnDocumentSession.SaveAsync, and the panel's CanEdit / Save-button gating and info alert. Top-level tasks in a process with subprocesses are now editable and saveable.
• Removed BpmnDefinitionsDocument.FindSubProcessIds and SubProcessElementType, which were dead code after the above.
• An element inside a subprocess is still not editable, because its body is not in the document. The message now names the server limitation (BPMN W34: elements inside subprocesses cannot be edited through the document endpoints elsa-core#8076) instead of implying a Studio one.
• The single-flight save and saving state from feat(workflows): bind BPMN tasks to Elsa activities from a Performed by section #1031 are untouched.

Compatibility

The document endpoints exist only in elsa-core 3.8 preview builds. Preview builds made between elsa-core #8060 and #8077 serve them without the fix, and saving from Studio against those builds would still empty subprocesses. No stable release is affected.

Tests

• Session: SaveAsync on a document with a subprocess sends the PUT.
• bUnit: in a document with a subprocess, a top-level task shows its editors and Save sends the PUT.
• bUnit: an element inside a subprocess is still not editable, and the message names the core limitation.
• Full Elsa.Studio.Workflows.Tests run: 445/445.
• A test-host crash in WorkflowInstanceDesignerDisconnectRefreshTests (from fix(workflows): stop instance designer refresh timers when the circuit disconnects #992) appeared once under heavy machine load. It did not reproduce in 2 runs on this branch or 2 runs on main.

🤖 Generated with Claude Code

[greptile-apps]

Confidence Score: 5/5

Safe to merge; there are no outstanding blocking issues.

Summary

• This PR enables saving BPMN documents containing subprocesses now that the server preserves their stored bodies, while keeping elements inside subprocesses read-only. It removes obsolete save guards and distinguishes unavailable nested-scope elements from stale or missing top-level selections.

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