diffusion: Fix silent override of explicit guidance_scale=0 (#4999)

Signed-off-by: Harenome Ranaivoarivony-Razanajato <harenome.razanajato1@huawei.com>

tests/diffusion/test_diffusion_request.py

@@ -72,3 +72,40 @@ def test_tp_seed_same_across_ranks_and_varies_across_requests():
 
     # Seeds must vary across requests (non-determinism preserved).
     assert len(set(seeds)) == n_requests, f"Expected {n_requests} unique seeds but got {len(set(seeds))}: {seeds}"
+
+
+def _request_with_guidance(**sampling_kwargs) -> OmniDiffusionRequest:
+    return OmniDiffusionRequest(
+        prompt={"prompt": "a cup of coffee on a table"},
+        sampling_params=OmniDiffusionSamplingParams(num_inference_steps=1, **sampling_kwargs),
+        request_id="request-test",
+    )
+
+
+def test_omitted_guidance_scale_is_unset_and_resolved_to_one():
+    # No guidance_scale passed: the caller did not set it, so it must NOT count as
+    # "provided" (each pipeline applies its own default), and __post_init__ resolves the
+    # None "unset" sentinel to a concrete float so no consumer ever observes None.
+    req = _request_with_guidance()
+
+    assert req.sampling_params.guidance_scale_provided is False
+    assert req.sampling_params.guidance_scale == 1.0
+
+
+def test_explicit_zero_guidance_scale_is_honored():
+    # Regression (#4998): an explicit guidance_scale=0 (disable CFG) must be honored,
+    # not mistaken for "unset" and overridden by the pipeline's model default. This is
+    # the sentinel collision that the None default + ``is not None`` check fixes.
+    req = _request_with_guidance(guidance_scale=0.0)
+
+    assert req.sampling_params.guidance_scale_provided is True
+    assert req.sampling_params.guidance_scale == 0.0
+
+
+@pytest.mark.parametrize("value", [0.999, 1.0, 5.0])
+def test_explicit_nonzero_guidance_scale_is_honored(value):
+    # Any explicitly-set value (truthy or not) is honored as provided and preserved.
+    req = _request_with_guidance(guidance_scale=value)
+
+    assert req.sampling_params.guidance_scale_provided is True
+    assert req.sampling_params.guidance_scale == value