Destroy Geant4 worker when all result files have been fetched

[kmichalikk]

Fixes #2291

[Copilot]

Pull request overview

This PR implements automatic cleanup of Geant4 workers after result files have been fetched, addressing issue #2291 to improve memory management. The implementation adds protection against concurrent hydration calls and gracefully handles worker destruction timing.

Key Changes

• Added early-exit check in hydrateResults to prevent re-hydration when results already exist
• Wrapped file fetching in try-catch block to handle worker destruction from concurrent calls
• Introduced markSafeForDeletion() method as the public API for worker cleanup, making destroy() private
• Called markSafeForDeletion() after successful result hydration to free worker resources

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/services/Geant4LocalWorkerSimulationService.ts	Added concurrent hydration protection and worker cleanup after results are stored; wrapped file fetching in try-catch for graceful error handling
src/Geant4Worker/Geant4Worker.ts	Introduced public markSafeForDeletion() method and made destroy() private to control worker lifecycle
src/Geant4Worker/Geant4DatasetDownloadManager.ts	Updated to use new markSafeForDeletion() API instead of directly calling destroy()

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