feat: Implement task-level Fusion override via disk directive

Enable per-task control of Fusion filesystem by checking the disk.fusion
setting before falling back to executor-level configuration.

Changes:
- Modify fusionEnabled() in FusionAwareTask to check task's disk.fusion
  setting first
- Fall back to executor.isFusionEnabled() if disk.fusion is not specified
- Task-level setting takes priority over global fusion.enabled config
- Add null-safe navigation for config to handle cases where task config
  may not be set (e.g., in test mocks)

Priority order:
1. Task-level disk.fusion setting (highest)
2. Executor/session-level fusion.enabled config (fallback)

This provides a clearer alternative to using "scratch false" for
disabling Fusion on problematic tasks.

Signed-off-by: Edmund Miller <edmund.miller@seqera.io>

Author: edmundmiller

modules/nextflow/src/main/groovy/nextflow/fusion/FusionAwareTask.groovy

@@ -42,7 +42,14 @@ trait FusionAwareTask {
 
     boolean fusionEnabled() {
         if( fusionEnabled==null ) {
-            fusionEnabled = getExecutor0().isFusionEnabled()
+            // Check task-level disk.fusion setting first
+            def diskFusion = getTask().config?.getDiskResource()?.fusion
+            if( diskFusion != null ) {
+                fusionEnabled = diskFusion
+            } else {
+                // Fall back to executor-level setting
+                fusionEnabled = getExecutor0().isFusionEnabled()
+            }
         }
         return fusionEnabled
     }