Add backward compatibility for GuidedDecodingParams (#25422)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

tests/v1/entrypoints/llm/test_struct_output_generate.py

@@ -5,6 +5,7 @@
 from __future__ import annotations
 
 import json
+from dataclasses import fields
 from enum import Enum
 from typing import TYPE_CHECKING, Any
 
@@ -21,7 +22,8 @@ from vllm.entrypoints.llm import LLM
 from vllm.outputs import RequestOutput
 from vllm.platforms import current_platform
 from vllm.reasoning.abs_reasoning_parsers import ReasoningParserManager
-from vllm.sampling_params import SamplingParams, StructuredOutputsParams
+from vllm.sampling_params import (GuidedDecodingParams, SamplingParams,
+                                  StructuredOutputsParams)
 
 if TYPE_CHECKING:
     from vllm.config import TokenizerMode
@@ -89,6 +91,26 @@ def _load_json(s: str, backend: str) -> str:
     return json.loads(s)
 
 
+def test_guided_decoding_deprecated():
+    with pytest.warns(DeprecationWarning,
+                      match="GuidedDecodingParams is deprecated.*"):
+        guided_decoding = GuidedDecodingParams(json_object=True)
+
+    structured_outputs = StructuredOutputsParams(json_object=True)
+    assert fields(guided_decoding) == fields(structured_outputs)
+
+    with pytest.warns(DeprecationWarning,
+                      match="guided_decoding is deprecated.*"):
+        sp1 = SamplingParams(guided_decoding=guided_decoding)
+
+    with pytest.warns(DeprecationWarning,
+                      match="guided_decoding is deprecated.*"):
+        sp2 = SamplingParams.from_optional(guided_decoding=guided_decoding)
+
+    assert sp1 == sp2
+    assert sp1.structured_outputs == guided_decoding
+
+
 @pytest.mark.skip_global_cleanup
 @pytest.mark.parametrize(
     "model_name, backend, tokenizer_mode, speculative_config",