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/Functions/ExplicitReturnInInit.ql

@@ -19,7 +19,9 @@ where
   exists(Function init | init.isInitMethod() and r.getScope() = init) and
   r.getValue() = rv and
   not rv.pointsTo(Value::none_()) and
-  not exists(FunctionValue f | f.getACall() = rv.getAFlowNode() | f.neverReturns()) and
+  not exists(FunctionValue f, ControlFlowNode rvCfg | rvCfg.getNode() = rv |
+    f.getACall() = rvCfg and f.neverReturns()
+  ) and
   // to avoid double reporting, don't trigger if returning result from other __init__ function
   not exists(Attribute meth | meth = rv.(Call).getFunc() | meth.getName() = "__init__")
 select r, "Explicit return in __init__ method."

python/ql/src/Functions/ReturnValueIgnored.ql

@@ -69,7 +69,12 @@ where
   returns_meaningful_value(callee) and
   not wrapped_in_try_except(call) and
   exists(int unused |
-    unused = count(ExprStmt e | e.getValue().getAFlowNode() = callee.getACall()) and
+    unused =
+      count(ExprStmt e |
+        exists(ControlFlowNode eValCfg | eValCfg.getNode() = e.getValue() |
+          eValCfg = callee.getACall()
+        )
+      ) and
     total = count(callee.getACall())
   |
     percentage_used = (100.0 * (total - unused) / total).floor()