Python: deprecate AstNode.getAFlowNode() and rewrite internal callers

Preparatory refactor for the shared-CFG dataflow migration.

Deprecates the AstNode.getAFlowNode() cached predicate on the public
Python QL API and rewrites all ~140 internal callers across lib/, src/,
test/, and tools/ from `expr.getAFlowNode() = cfgNode` to
`cfgNode.getNode() = expr`, using ControlFlowNode.getNode() which
already exists in Flow.qll.

The predicate itself is preserved (with a deprecation note pointing at
the new pattern) so external users do not experience churn — they can
migrate at their own pace and the AST/CFG hierarchies still get the
intended untangling once the deprecation eventually elapses.

Semantic noop verified by:
- All 361 lib/ + src/ queries compile clean.
- All 122 ControlFlow + PointsTo library-tests pass.
- All 64 dataflow library-tests pass.
- All 113 Variables/Exceptions/Expressions/Statements/Functions/Imports/
  Security/CWE-798/ModificationOfParameterWithDefault query-tests pass.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/ql/src/Classes/ClassAttributes.qll

@@ -48,9 +48,11 @@ class CheckClass extends ClassObject {
         self_dict = sub.getObject()
         or
         /* Indirect assignment via temporary variable */
-        exists(SsaVariable v |
-          v.getAUse() = sub.getObject().getAFlowNode() and
-          v.getDefinition().(DefinitionNode).getValue() = self_dict.getAFlowNode()
+        exists(SsaVariable v, ControlFlowNode subObjCfg, ControlFlowNode selfDictCfg |
+          subObjCfg.getNode() = sub.getObject() and selfDictCfg.getNode() = self_dict
+        |
+          v.getAUse() = subObjCfg and
+          v.getDefinition().(DefinitionNode).getValue() = selfDictCfg
         )
       ) and
       a.getATarget() = sub and
@@ -62,9 +64,10 @@ class CheckClass extends ClassObject {
 
   pragma[nomagic]
   private predicate monkeyPatched(string name) {
-    exists(Attribute a |
+    exists(Attribute a, ControlFlowNode objCfg |
+      objCfg.getNode() = a.getObject() and
       a.getCtx() instanceof Store and
-      PointsTo::points_to(a.getObject().getAFlowNode(), _, this, _, _) and
+      PointsTo::points_to(objCfg, _, this, _, _) and
       a.getName() = name
     )
   }
@@ -84,9 +87,9 @@ class CheckClass extends ClassObject {
   }
 
   predicate interestingUndefined(SelfAttributeRead a) {
-    exists(string name | name = a.getName() |
+    exists(string name, ControlFlowNode aCfg | name = a.getName() and aCfg.getNode() = a |
       this.interestingContext(a, name) and
-      not this.definedInBlock(a.getAFlowNode().getBasicBlock(), name)
+      not this.definedInBlock(aCfg.getBasicBlock(), name)
     )
   }
 
@@ -109,8 +112,9 @@ class CheckClass extends ClassObject {
 
   pragma[nomagic]
   private predicate definitionInBlock(BasicBlock b, string name) {
-    exists(SelfAttributeStore sa |
-      sa.getAFlowNode().getBasicBlock() = b and
+    exists(SelfAttributeStore sa, ControlFlowNode saCfg |
+      saCfg.getNode() = sa and
+      saCfg.getBasicBlock() = b and
       sa.getName() = name and
       sa.getClass() = this.getPyClass()
     )