Fix ModuleNotFoundError for missing TTS package in khanomtan_tts

[Copilot]

khanomtan_tts.py had a top-level from TTS.utils.synthesizer import Synthesizer import, causing an immediate ModuleNotFoundError for any user without coqui-tts installed — even when using unrelated pretrained models like lunarlist_onnx.

Changes

• pythaitts/pretrained/khanomtan_tts.py: Moved Synthesizer import from module-level into load_synthesizer(), wrapped in try/except ImportError with a clear install hint. Matches the existing lazy-import pattern in lunarlist_model.py.

def load_synthesizer(self, mode):
    try:
        from TTS.utils.synthesizer import Synthesizer
    except ImportError:
        raise ImportError(
            "You must install coqui-tts before using this model: pip install coqui-tts"
        )

• tests/test_khanomtan.py: Adds a test verifying the ImportError is raised with the expected message when the TTS package is absent.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>ModuleNotFoundError: No module named 'TTS'</issue_title>
<issue_description>Code raises error when trying to follow the official snippets.

/Users/sarit/.pyenv/versions/learn_typhoon_ocr/bin/python /Users/sarit/fullstack/learn_typhoon_ocr/test_pythaitts.py 
Traceback (most recent call last):
  File "/Users/sarit/fullstack/learn_typhoon_ocr/test_pythaitts.py", line 34, in <module>
    tts = TTS(pretrained="khanomtan")
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/sarit/.pyenv/versions/learn_typhoon_ocr/lib/python3.12/site-packages/pythaitts/__init__.py", line 40, in __init__
    self.load_pretrained(version=version)
  File "/Users/sarit/.pyenv/versions/learn_typhoon_ocr/lib/python3.12/site-packages/pythaitts/__init__.py", line 50, in load_pretrained
    from pythaitts.pretrained.khanomtan_tts import KhanomTan
  File "/Users/sarit/.pyenv/versions/learn_typhoon_ocr/lib/python3.12/site-packages/pythaitts/pretrained/khanomtan_tts.py", line 12, in <module>
    from TTS.utils.synthesizer import Synthesizer
ModuleNotFoundError: No module named 'TTS'

Process finished with exit code 1

import sounddevice as sd
import soundfile as sf
from pythaitts import TTS

tts = TTS()

tts = TTS(pretrained="khanomtan")
file = tts.tts("ภาษาไทย", speaker_idx="Linda", filename="kanontan.wav")
data, samplerate = sf.read(file)
print(f"เล่นจากไฟล์: {file}")
sd.play(data, samplerate)
sd.wait()  # รอจนเล่นเสร็จ

pythaitts==0.4.2

</issue_description>

Comments on the Issue (you are @copilot in this section)

@wannaphong Thank you for you report! It is weird bug because I still can use pythaitts in [google colab](https://colab.research.google.com/github/PyThaiNLP/PyThaiTTS/blob/dev/notebook/use_lunarlist_model.ipynb).

• Fixes ModuleNotFoundError: No module named 'TTS' #23

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