Python: treat await expressions as side-effecting

Prevent ineffectual-statement false positives when awaiting a task or coroutine without assigning its returned value. Add regression cases for both forms.

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

Copilot-Session: f0800d30-e4f9-463c-a0d8-660fe1aceaf1

Author: baywet

python/ql/lib/change-notes/2026-11-05-await-side-effects.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* Await expressions are now treated as having side effects. This prevents false positives from the "Statement has no effect" query (`py/ineffectual-statement`) when an awaited task or coroutine's result is discarded.

python/ql/lib/semmle/python/Exprs.qll

@@ -623,6 +623,8 @@ class None extends NameConstant {
 class Await extends Await_ {
   /* syntax: await Expr */
   override Expr getASubExpression() { result = this.getValue() }
+
+  override predicate hasSideEffects() { any() }
 }
 
 /** A formatted string literal expression, such as `f'hello {world!s}'` */

python/ql/test/query-tests/Statements/no_effect/await_task.py

@@ -0,0 +1,15 @@
+import asyncio
+
+
+async def returns_value():
+    return 42
+
+
+async def await_task_result():
+    task = asyncio.create_task(returns_value())
+    await task
+
+
+async def await_coroutine_result():
+    coroutine = returns_value()
+    await coroutine