Python: include try-else in getChild for completion propagation

The shared CFG library propagates abrupt completions from child to
parent via getChild(parent, _) = child. Python's try.getElse() was
wired into normal step rules but not listed in getChild(TryStmt, ...),
so return/break/continue/raise statements occurring inside a try-else
block had no parent path and ended up as dead-end CFG nodes.

Add the else block at index -2 (alongside finally at -1). This affects
only completion propagation; the normal-flow CFG is unchanged because
TryStmt has explicit step rules.

Verified on a CPython database: all 11 shared-CFG consistency queries
now pass with 0 violations (deadEnd: 244 -> 0).

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

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

@@ -969,13 +969,15 @@ module Ast implements AstSig<Py::Location> {
       index = 1 and result = r.getCause()
     )
     or
-    // TryStmt: body (0), handlers (1..n), finally (-1)
+    // TryStmt: body (0), handlers (1..n), else (-2), finally (-1)
     exists(TryStmt t | t = n |
       index = 0 and result = t.getBody()
       or
       result = t.getCatch(index - 1) and index >= 1
       or
       index = -1 and result = t.getFinally()
+      or
+      index = -2 and result = t.getElse()
     )
     or
     // Switch (match): subject (0), cases (1..n)