Remove hollow unit test, strengthen e2e with semantic checks

The unit test only verified that a Python if/else branches correctly —
it was a copy of the production logic, not a test of it. Replace with
a stronger e2e assertion: raw_logprobs must be <= 0 (log_softmax
guarantee), and the two modes must produce different values.

Co-authored-by: Federico Kamelhar <209537060+fede-kamel@users.noreply.github.com>
Co-authored-by: Allen Shen <aoshen@inferact.ai>
Signed-off-by: Allen Shen <aoshen@inferact.ai>

Author: aoshen02

tests/v1/sample/test_prompt_logprobs_mode.py

@@ -185,7 +185,8 @@ def test_seed(llm):
 
 
 def test_prompt_logprobs_respects_logprobs_mode():
-    """prompt_logprobs values must differ between raw_logits and raw_logprobs."""
+    """raw_logits must return positive logits, raw_logprobs must return
+    negative log_softmax values, and both must differ."""
     prompt = "Hello world"
     values: dict[str, float] = {}
 
@@ -201,6 +202,12 @@ def test_prompt_logprobs_respects_logprobs_mode():
         values[mode] = out.prompt_logprobs[1][tok_id].logprob
         del engine
 
+    # log_softmax values are always <= 0; raw logits for the top token
+    # are typically positive. This verifies the mode actually changed
+    # the computation, not just that two floats differ.
+    assert values["raw_logprobs"] <= 0, (
+        f"raw_logprobs should be <= 0, got {values['raw_logprobs']}"
+    )
     assert values["raw_logits"] != values["raw_logprobs"], (
         "prompt_logprobs should differ between logits and logprobs modes"
     )