Fix test_no_optuna reinstalling optuna at wrong version

test_no_optuna() uninstalls optuna to test graceful handling, then
reinstalls it. It was hardcoded to reinstall optuna==2.8.0 instead of
the version range used elsewhere (>=2.8.0,<=3.6.1), leaving the test
environment with a downgraded optuna for the rest of the CI run. This
caused the saved dependency snapshot to incorrectly report optuna==2.8.0.

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

Author: thinkall

test/tune/test_searcher.py

@@ -323,7 +323,7 @@ def test_no_optuna():
     subprocess.check_call([sys.executable, "-m", "pip", "uninstall", "-y", "optuna"])
     import flaml.tune.searcher.suggestion
 
-    subprocess.check_call([sys.executable, "-m", "pip", "install", "optuna==2.8.0"])
+    subprocess.check_call([sys.executable, "-m", "pip", "install", "optuna>=2.8.0,<=3.6.1"])
 
 
 def test_unresolved_search_space(caplog):