[Misc] Add SPDX-FileCopyrightText (#19100)

Signed-off-by: simon-mo <simon.mo@hey.com>

tests/spec_decode/test_memory_usage.py

@@ -1,4 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 """This docstring details important information on the testing methodology.
 
 This test verifies that memory usage remains constant (or never grows) when