fix: split long text into chunks to prevent distortion on long inputs (#372)

[ousamabenyounes]

Long single-shot generations degrade into distortion past a few hundred characters (worse on VoxCPM2). VoxCPM._generate now splits long text on sentence boundaries into <=max_chunk_chars chunks and synthesizes each with the same prompt cache, keeping every generation short while preserving the voice.

Test verification (RED → GREEN)

With the fix reverted, the new test fails (RED):

        text = "这是一段很长的测试文本。" * 10  # ~120 chars
    
>       out = list(vox._generate(text, streaming=False, max_chunk_chars=MAX_CHARS))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E       TypeError: VoxCPM._generate() got an unexpected keyword argument 'max_chunk_chars'

tests/test_long_text_chunking.py:153: TypeError
___________________ test_generate_short_text_is_single_call ____________________

    def test_generate_short_text_is_single_call():
        model = _RecordingModel()
        vox = _make_vox(model)
        text = "短文本。"
    
>       list(vox._generate(text, streaming=False, max_chunk_chars=MAX_CHARS))
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E       TypeError: VoxCPM._generate() got an unexpected keyword argument 'max_chunk_chars'

tests/test_long_text_chunking.py:170: TypeError
__________________ test_generate_streaming_yields_each_chunk ___________________

    def test_generate_streaming_yields_each_chunk():
        model = _RecordingModel()
        vox = _make_vox(model)
        text = "流式测试的句子。" * 8
    
>       out = list(vox._generate(text, streaming=True, max_chunk_chars=MAX_CHARS))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E       TypeError: VoxCPM._generate() got an unexpected keyword argument 'max_chunk_chars'

tests/test_long_text_chunking.py:181: TypeError
=========================== short test summary info ============================
FAILED tests/test_long_text_chunking.py::test_short_text_is_single_chunk - At...
FAILED tests/test_long_text_chunking.py::test_long_text_splits_on_sentence_boundaries
FAILED tests/test_long_text_chunking.py::test_splitting_disabled_when_max_chars_non_positive
FAILED tests/test_long_text_chunking.py::test_sentence_without_delimiter_is_hard_sliced
FAILED tests/test_long_text_chunking.py::test_generate_chunks_long_text_into_multiple_calls
FAILED tests/test_long_text_chunking.py::test_generate_short_text_is_single_call
FAILED tests/test_long_text_chunking.py::test_generate_streaming_yields_each_chunk
============================== 7 failed in 0.08s ===============================

With the fix applied, the test passes (GREEN):

GREEN attempt 1/2 (exit 0)
============================= test session starts ==============================
platform linux -- Python 3.14.4, pytest-9.0.2, pluggy-1.6.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /home/ousama/contribute-work/OpenBMB__VoxCPM
configfile: pyproject.toml
plugins: typeguard-4.4.4
collecting ... collected 7 items

tests/test_long_text_chunking.py::test_short_text_is_single_chunk PASSED
tests/test_long_text_chunking.py::test_long_text_splits_on_sentence_boundaries PASSED
tests/test_long_text_chunking.py::test_splitting_disabled_when_max_chars_non_positive PASSED
tests/test_long_text_chunking.py::test_sentence_without_delimiter_is_hard_sliced PASSED
tests/test_long_text_chunking.py::test_generate_chunks_long_text_into_multiple_calls long text -> 4 chunks, audio samples=120
PASSED
tests/test_long_text_chunking.py::test_generate_short_text_is_single_call short text -> 1 chunk
PASSED
tests/test_long_text_chunking.py::test_generate_streaming_yields_each_chunk streaming long text -> 2 chunks yielded
PASSED

============================== 7 passed in 0.06s ===============================

Full local suite

Command: python3 -m pytest tests/

============================= test session starts ==============================
platform linux -- Python 3.14.4, pytest-9.0.2, pluggy-1.6.0
rootdir: /home/ousama/contribute-work/OpenBMB__VoxCPM
configfile: pyproject.toml
plugins: typeguard-4.4.4
collected 52 items / 2 errors

==================================== ERRORS ====================================
____________ ERROR collecting tests/test_lora_checkpoint_loading.py ____________
ImportError while importing test module '/home/ousama/contribute-work/OpenBMB__VoxCPM/tests/test_lora_checkpoint_loading.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.14/importlib/__init__.py:88: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
tests/test_lora_checkpoint_loading.py:9: in <module>
    import torch
E   ModuleNotFoundError: No module named 'torch'
__________________ ERROR collecting tests/test_model_utils.py __________________
ImportError while importing test module '/home/ousama/contribute-work/OpenBMB__VoxCPM/tests/test_model_utils.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.14/importlib/__init__.py:88: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
tests/test_model_utils.py:20: in <module>
    spec.loader.exec_module(utils)
src/voxcpm/model/utils.py:3: in <module>
    import torch
E   ModuleNotFoundError: No module named 'torch'
=========================== short test summary info ============================
ERROR tests/test_lora_checkpoint_loading.py
ERROR tests/test_model_utils.py
!!!!!!!!!!!!!!!!!!! Interrupted: 2 errors during collection !!!!!!!!!!!!!!!!!!!!
============================== 2 errors in 0.16s ===============================

Fix #372