tasks: fix task state locking

Race condition iexisted where task State, err, and processReturnValue fields
were written by consumer goroutine and read by concurrent accessors without
proper synchronization, causing torn reads and data races.

Implemented single-lock model with optimal lock scope:

- Removed 8 accessor methods (direct field access is simpler)
- Lock only during brief state transitions (IDLE→RUNNING, RUNNING→SUCCEEDED/FAILED)
- Release lock during task.process() execution to enable full concurrency
- Readers hold list.Lock() only during atomic struct copy
- Moved State = RUNNING before goroutine spawn for clearer semantics
- task/list.go: RunTaskInBackground() copies *task before unlock,
    returns the pre-made copy instead of dereferencing after unlock

task/task.go

@@ -42,6 +42,7 @@ const (
 )
 
 // Task represents as task in a queue encapsulates process code
+// All fields are protected by List.Mutex - access task fields only while holding list.Lock()
 type Task struct {
 	output             *Output
 	detail             *Detail