[BugFix] Work-around incremental detokenization edge case error (#19449)

Signed-off-by: Nick Hill <nhill@redhat.com>

tests/v1/engine/test_fast_incdec_prefix_err.py

@@ -0,0 +1,80 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+from transformers import AutoTokenizer
+
+from vllm.sampling_params import SamplingParams
+from vllm.v1.engine import EngineCoreRequest
+from vllm.v1.engine.detokenizer import IncrementalDetokenizer
+
+# ruff: noqa: E501
+
+
+def test_fast_inc_detok_invalid_utf8_err_case():
+    """
+    Test edge case where tokenizer can produce non-monotonic,
+    invalid UTF-8 output, which breaks the internal state of
+    tokenizers' DecodeStream.
+    See https://github.com/vllm-project/vllm/issues/17448.
+
+    Thanks to reproducer from @fpaupier:
+    https://gist.github.com/fpaupier/0ed1375bd7633c5be6c894b1c7ac1be3.
+    """
+    tokenizer = AutoTokenizer.from_pretrained("google/gemma-3-1b-it")
+
+    # Create a test request
+    prompt_token_ids = [107, 4606, 236787, 107]
+    params = SamplingParams(skip_special_tokens=True)
+    request = EngineCoreRequest(
+        "test",
+        prompt_token_ids,
+        None,
+        None,
+        None,
+        params,
+        None,
+        0.0,
+        None,
+        cache_salt=None,
+        data_parallel_rank=None,
+    )
+
+    detokenizer = IncrementalDetokenizer.from_new_request(tokenizer, request)
+
+    assert detokenizer.__class__.__name__ == "FastIncrementalDetokenizer", \
+        "Should use FastIncrementalDetokenizer by default"
+
+    # Process tokens incrementally
+    test_tokens = [
+        236840, 107, 138, 236782, 107, 140, 236775, 6265, 1083, 623, 121908,
+        147418, 827, 107, 140, 236775, 6265, 236779, 2084, 1083, 623, 203292,
+        827, 107, 140, 236775, 6265, 236779, 7777, 1083, 623, 121908, 147418,
+        569, 537, 236789, 65880, 569, 537, 236789, 62580, 853, 115693, 210118,
+        35178, 16055, 1270, 759, 215817, 4758, 1925, 1117, 827, 107, 140,
+        236775, 5654, 1083, 623, 110733, 46291, 827, 107, 140, 236775, 5654,
+        236779, 2084, 1083, 623, 136955, 56731, 827, 107, 140, 236775, 5654,
+        236779, 7777, 1083, 623, 194776, 2947, 496, 109811, 1608, 890, 215817,
+        4758, 1925, 1117, 2789, 432, 398, 602, 31118, 569, 124866, 134772, 509,
+        19478, 1640, 33779, 236743, 236770, 236819, 236825, 236771, 432, 398,
+        432, 237167, 827, 107, 140, 236775, 77984, 1083, 623, 2709, 236745,
+        2555, 513, 236789, 602, 31118, 569
+    ]
+
+    output = ""
+    for i, token_id in enumerate(test_tokens):
+        detokenizer.update([token_id], False)
+
+        finished = i == len(test_tokens) - 1
+        output += detokenizer.get_next_output_text(finished, delta=True)
+
+
+# fmt: off
+    assert output == r'''[
+  {
+    "source": "Résultats",
+    "source_type": "CONCEPT",
+    "source_description": "Résultats de l'analyse de l'impact des opérations israéliennes sur la frontière libanaise",
+    "target": "Israël",
+    "target_type": "ORGANIZATION",
+    "target_description": "Pays qui a obtenu à sa frontière libanaise « un niveau de calme inédit depuis les années 1960 »",
+    "relationship": "Obtention d'un niveau de'''