Fix KeyError when transliterating text containing obsolete Thai letter ฅ

[Copilot]

What does this changes

Adds preprocessing to replace obsolete Thai letter ฅ (Kho Khon) with ค (Kho Khwai) before calling tltk library functions.

What was wrong

The tltk library throws KeyError: 'ฅ' when encountering the obsolete Thai character ฅ because it's not in tltk's vocabulary. This occurs in g2p(), th2ipa(), and th2roman() functions.

transliterate("ภาษาไวคาลีฅ", engine="tltk_g2p")
# KeyError: 'ฅ' at tltk/nlp.py line 1929

How this fixes it

• Modified romanize(), tltk_g2p(), and tltk_ipa() in pythainlp/transliterate/tltk.py to replace ฅ with ค before processing
• Both characters are phonetically identical, so transliteration output is unaffected
• Added test coverage for the ฅ character in tests/extra/testx_transliterate.py

Your checklist for this pull request

• Passed code styles and structures
• Passed code linting checks and unit test

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Transliterator cannot handle ฅ</issue_title>
<issue_description>transliterate sometimes throws a KeyError when the input contains the character ฅ.

In the following output, the word ภาษาไวคาลีฅ is taken from the list of processed Wikipedia article titles in this repo.

>>> transliterate("ภาษาไวคาลีฅ", engine="tltk_g2p")
Traceback (most recent call last):
  File "<python-input-7>", line 1, in <module>
    transliterate("ภาษาไวคาลีฅ", engine="tltk_g2p")
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/<SNIP>/venv/lib/python3.13/site-packages/pythainlp/transliterate/core.py", line 185, in transliterate
    return transliterate(text)
  File "/<SNIP>/venv/lib/python3.13/site-packages/pythainlp/transliterate/tltk.py", line 24, in tltk_g2p
    _temp = g2p(text).split("<tr/>")[1].replace("|<s/>", "").replace("|", " ")
            ~~~^^^^^^
  File "/<SNIP>/venv/lib/python3.13/site-packages/tltk/nlp.py", line 1690, in g2p
    y = sylparse(inp)
  File "/<SNIP>/venv/lib/python3.13/site-packages/tltk/nlp.py", line 1767, in sylparse
    phone = ReplaceSnd(PronF,codematch,charmatch)
  File "/<SNIP>/venv/lib/python3.13/site-packages/tltk/nlp.py", line 1929, in ReplaceSnd
    s = stable[x][tmp1Lst[i]]
        ~~~~~~~~~^^^^^^^^^^^^
KeyError: 'ฅ'

For now, this can be solved easily with this basic replacement: text.replace("ฅ", "ค"), which will not affect the transliteration.</issue_description>

<agent_instructions>Fix pythainlp/transliterate/tltk.py</agent_instructions>

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

@wannaphong Thank you for your report! It is tltk's issue because it is out-of-vocabulary(OOV) problem.

• Fixes Transliterator cannot handle ฅ #1176

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[coveralls]

coverage: 52.587% (-0.02%) from 52.606%
when pulling 621e15d on copilot/fix-transliterator-key-error
into 2d1466a on dev.