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/lib/semmle/python/objects/Callables.qll

@@ -56,8 +56,9 @@ abstract class CallableObjectInternal extends ObjectInternal {
 /** A Python function. */
 class PythonFunctionObjectInternal extends CallableObjectInternal, TPythonFunctionObject {
   override Function getScope() {
-    exists(CallableExpr expr |
-      this = TPythonFunctionObject(expr.getAFlowNode()) and
+    exists(CallableExpr expr, ControlFlowNode exprCfg |
+      exprCfg.getNode() = expr and
+      this = TPythonFunctionObject(exprCfg) and
       result = expr.getInnerScope()
     )
   }
@@ -160,10 +161,11 @@ class PythonFunctionObjectInternal extends CallableObjectInternal, TPythonFuncti
 }
 
 private BasicBlock blockReturningNone(Function func) {
-  exists(Return ret |
+  exists(Return ret, ControlFlowNode ret_ |
     not exists(ret.getValue()) and
     ret.getScope() = func and
-    result = ret.getAFlowNode().getBasicBlock()
+    ret_.getNode() = ret and
+    result = ret_.getBasicBlock()
   )
 }
 

python/ql/lib/semmle/python/objects/Classes.qll

@@ -113,8 +113,9 @@ abstract class ClassObjectInternal extends ObjectInternal {
 class PythonClassObjectInternal extends ClassObjectInternal, TPythonClassObject {
   /** Gets the scope for this Python class */
   Class getScope() {
-    exists(ClassExpr expr |
-      this = TPythonClassObject(expr.getAFlowNode()) and
+    exists(ClassExpr expr, ControlFlowNode exprCfg |
+      exprCfg.getNode() = expr and
+      this = TPythonClassObject(exprCfg) and
       result = expr.getInnerScope()
     )
   }

python/ql/lib/semmle/python/objects/TObject.qll

@@ -387,7 +387,7 @@ private PythonClassObjectInternal abcMetaClassObject() {
 private predicate neither_class_nor_static_method(Function f) {
   not exists(f.getADecorator())
   or
-  exists(ControlFlowNode deco | deco = f.getADecorator().getAFlowNode() |
+  exists(ControlFlowNode deco | deco.getNode() = f.getADecorator() |
     exists(ObjectInternal o | PointsToInternal::pointsTo(deco, _, o, _) |
       o != ObjectInternal::staticMethod() and
       o != ObjectInternal::classMethod()