[Chore] Cleanup guided namespace, move to structured outputs config (#22772)

Signed-off-by: Aaron Pham <contact@aarnphm.xyz>
Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>
Co-authored-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

tests/entrypoints/openai/test_completion_with_function_calling.py

@@ -142,7 +142,7 @@ def server():  # noqa: F811
         "--dtype",
         "half",
         "--enable-auto-tool-choice",
-        "--guided-decoding-backend",
+        "--structured-outputs-config.backend",
         "xgrammar",
         "--tool-call-parser",
         "hermes",
@@ -225,7 +225,7 @@ def k2_server():  # noqa: F811
         "--dtype",
         "half",
         "--enable-auto-tool-choice",
-        "--guided-decoding-backend",
+        "--structured-outputs-config.backend",
         "xgrammar",
         "--tool-call-parser",
         "hermes",