Cache Python shared CFG node mapping

The canonical injects-to-Python-AST mapping is evaluated repeatedly across DCA prewarm and target stages. Cache getNode so this truthful mapping forms a reusable evaluation boundary without changing its semantics.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 529363f5-bc7d-4f0b-9f47-e03ba9aa0cdf

Author: yoff

python/ql/lib/semmle/python/controlflow/internal/Cfg.qll

@@ -35,6 +35,7 @@ private import codeql.controlflow.SuccessorType
  */
 class ControlFlowNode extends CfgImpl::ControlFlowNode {
   /** Gets the syntactic element corresponding to this flow node, if any. */
+  cached
   Py::AstNode getNode() {
     exists(CfgImpl::Ast::AstNode n | this.injects(n) | result = CfgImpl::astNodeToPyNode(n))
   }