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/Statements/NestedLoopsSameVariableWithReuse.ql

@@ -15,7 +15,7 @@
 import python
 
 predicate loop_variable_ssa(For f, Variable v, SsaVariable s) {
-  f.getTarget().getAFlowNode() = s.getDefinition() and v = s.getVariable()
+  s.getDefinition().getNode() = f.getTarget() and v = s.getVariable()
 }
 
 predicate variableUsedInNestedLoops(For inner, For outer, Variable v, Name n) {