[Test] Make `test_failure` more stable for batch invariance (#27054)

tests/v1/generation/test_batch_invariance.py

@@ -59,12 +59,15 @@ def _random_prompt(min_words: int = 1024, max_words: int = 1024 * 2) -> str:
     # Pick a random template
     base_prompt = random.choice(prompt_templates)
 
-    # Add some padding to vary the length if needed
-    if min_words > 50:
+    if max_words < min_words:
+        max_words = min_words
+    target_words = random.randint(min_words, max_words)
+
+    if target_words > 50:
         # For longer prompts, repeat context
         padding_text = (
             " This is an interesting topic that deserves more explanation. "
-            * (min_words // 50)
+            * (target_words // 50)
         )
         base_prompt = base_prompt + padding_text
 
@@ -516,8 +519,20 @@ def test_logprobs_WITHOUT_batch_invariance_should_FAIL(backend):
             dtype="bfloat16",
         )
 
-        # Use more realistic prompts for better token generation
-        prompts = [_random_prompt(10, 50) for i in range(32)]
+        # build ragged prompts to change shapes significantly across BS=1 vs BS=N
+        long_min = int(os.getenv("VLLM_MIN_PROMPT", "768"))
+        long_max = int(os.getenv("VLLM_MAX_PROMPT", "2048"))
+        prompts: list[str] = []
+        options = [
+            (max(long_min, 1536), max(long_max, 3072)),  # very long
+            (max(1024, long_min), max(2048, long_max)),  # long
+            (256, 512),  # mid
+            (10, 20),  # short
+        ]
+
+        for _ in range(32):
+            lo, hi = random.choice(options)
+            prompts.append(_random_prompt(lo, hi))
 
         sp = SamplingParams(
             temperature=0.6,