Python: fold in evaluation-order review-comment fixes from main

After rebasing onto main, apply the substance of upstream review-comment
commits (1ef557c972, 35faec3db1):

- timer.py: stricter validation (raise TypeError for unknown subscript
  elements), bypass atexit via os._exit on failure.
- test_basic.py: simpler test cases per review (drop unnecessary parens,
  use call form in test_callable_syntax), updated docstring.
- TimerUtils.qll: docstring update reflecting the t[dead(n)] / t[never]
  forms.

The 'dead(2)' annotation in test_boolean.py:27 is kept because our
NewCfgBranchTimestamps check (added on this branch) requires it.

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

python/ql/test/library-tests/ControlFlow/evaluation-order/TimerUtils.qll

@@ -1,9 +1,10 @@
 /**
  * Utility library for identifying timer annotations in evaluation-order tests.
  *
- * Identifies `expr @ t[n]` (matmul), `t(expr, n)` (call), and
- * `expr @ t.dead[n]` (dead-code) patterns, extracts timestamp values,
- * and provides predicates for traversing consecutive annotated CFG nodes.
+ * Identifies `expr @ t[n]` (matmul) and `t(expr, n)` (call) patterns,
+ * including `dead(n)` and `never` markers within subscripts, extracts
+ * timestamp values, and provides predicates for traversing consecutive
+ * annotated CFG nodes.
  */
 
 import python

python/ql/test/library-tests/ControlFlow/evaluation-order/test_basic.py

@@ -5,7 +5,7 @@ are evaluated in the expected order (typically left to right for
 operands of binary operators, elements of collection literals, etc.)
 
 Every evaluated expression has a timestamp annotation, except the
-timer mechanism itself (t[n], t.dead[n]).
+timer mechanism itself (t[n], t[dead(n)], t[never]).
 """
 
 from timer import test, never
@@ -46,7 +46,7 @@ def test_nested_binary(t):
 @test
 def test_chained_add(t):
     """a + b + c is (a + b) + c: left to right."""
-    x = ((1 @ t[0] + 2 @ t[1]) @ t[2] + 3 @ t[3]) @ t[4]
+    x = (1 @ t[0] + 2 @ t[1] + 3 @ t[2]) @ t[3]
 
 
 @test
@@ -58,7 +58,7 @@ def test_mixed_precedence(t):
 @test
 def test_string_concat(t):
     """String concatenation operands: left to right."""
-    x = (("hello" @ t[0] + " " @ t[1]) @ t[2] + "world" @ t[3]) @ t[4]
+    x = ("hello" @ t[0] + " " @ t[1] + "world" @ t[2]) @ t[3]
 
 
 @test
@@ -142,8 +142,8 @@ def test_multiple_assignment(t):
 @test
 def test_callable_syntax(t):
     """t(value, n) is equivalent to value @ t[n]."""
-    x = (1 @ t[0] + 2 @ t[1]) @ t[2]
-    y = (x @ t[3] * 3 @ t[4]) @ t[5]
+    x = t(t(1, 0) + t(2, 1), 2)
+    y = t(t(x, 3) * t(3, 4), 5)
 
 
 @test

python/ql/test/library-tests/ControlFlow/evaluation-order/timer.py

@@ -26,6 +26,7 @@ Run a test file directly to self-validate: python test_file.py
 """
 
 import atexit
+import os
 import sys
 
 _results = []
@@ -53,6 +54,10 @@ class _Check:
                 self._dead.add(e.timestamp)
             elif isinstance(e, _NeverSentinel):
                 self._never = True
+            else:
+                raise TypeError(
+                    f"Unknown element in timer subscript: {e!r} (type {type(e).__name__})"
+                )
 
     def __rmatmul__(self, value):
         ts = self._timer._tick()
@@ -183,7 +188,7 @@ def _report():
     print("---")
     print(f"{passed}/{total} tests passed")
     if passed < total:
-        sys.exit(1)
+        os._exit(1)
 
 
 atexit.register(_report)