Restore DRMAA (GridExecutor) as primary cluster execution path

The run() function was changed in 0.6.18 to always use get_executor(),
which routes all cluster jobs through the new CLI-based SlurmExecutor/
SGEExecutor classes, bypassing GridExecutor (DRMAA) entirely.

This broke all users with a working DRMAA installation.

Fix:
- run() now uses GridExecutor when DRMAA is available and a session is
  active; CLI-based executors are used only as a fallback when DRMAA is
  absent.
- will_run_on_cluster() no longer raises when DRMAA is missing; it
  returns False so the CLI executor fallback can proceed cleanly.

Author: Acribbs

cgatcore/pipeline/execution.py

@@ -540,17 +540,14 @@ def will_run_on_cluster(options):
     # First check if we're explicitly running without cluster
     if options.get("without_cluster", False):
         return False
-        
+
     wants_cluster = options.get("to_cluster", True)
-    
-    # Only raise error if DRMAA is required but not available
-    if wants_cluster and not HAS_DRMAA:
-        raise ValueError(
-            "Cluster execution requested (to_cluster=True) but DRMAA library is not available. "
-            "Please install drmaa package in your environment to enable cluster execution.")
-    
-    return (wants_cluster 
-            and HAS_DRMAA 
+
+    # Return True only when DRMAA is present and a session is active.
+    # When DRMAA is absent the CLI-based executors in get_executor() are used
+    # as a fallback, so we do not raise here.
+    return (wants_cluster
+            and HAS_DRMAA
             and GLOBAL_SESSION is not None)
 
 
@@ -1522,8 +1519,16 @@ def run(statement, **kwargs):
             logger.info("Dry-run: {}".format(statement))
         return []
 
-    # Use get_executor to get the appropriate executor
-    executor = get_executor(options)  # Updated to use get_executor
+    # Prefer DRMAA (GridExecutor) when available - it is the stable, battle-tested
+    # path.  Fall back to the CLI-based executors only when DRMAA is absent.
+    wants_cluster = (
+        options.get("to_cluster", True)
+        and not options.get("without_cluster", False)
+    )
+    if wants_cluster and HAS_DRMAA and GLOBAL_SESSION is not None:
+        executor = GridExecutor(**options)
+    else:
+        executor = get_executor(options)
 
     # Execute statement list within the context of the executor
     with executor as e: