Add None validation for corpus paths to prevent confusing runtime errors

[Copilot]

What does this changes

Adds defensive validation for corpus file paths that can be None when files aren't downloaded, preventing confusing TypeError and AttributeError at runtime. Adds get_thwiki_lstm() helper for validated access. Fixes resource leak in test.

What was wrong

get_corpus_path() returns None when corpus files aren't downloaded, but code assumed non-None:

# ulmfit/core.py - dict values could be None
THWIKI_LSTM = {
    "wgts_fname": get_corpus_path(_MODEL_NAME_LSTM),  # Could be None
    "itos_fname": get_corpus_path(_ITOS_NAME_LSTM),   # Could be None
}

# generate/thai2fit.py - TypeError: expected str, not NoneType
with open(thwiki["itos_fname"], "rb") as f:  # Crashes if None

# tag/thainer.py - passes None to pycrfsuite
self.crf.open(get_corpus_path("thainer-1.4"))  # Crashes if None

# tests/extra/testx_ulmfit.py - resource leak
pickle.load(open(thwiki["itos_fname"], "rb"))  # File never closed

How this fixes it

Validation with clear errors:

• ulmfit/core.py: Added get_thwiki_lstm() that validates paths and raises RuntimeError with download instructions. THWIKI_LSTM dict preserved for backward compatibility.
• generate/thai2fit.py: Validates paths at module load before file operations
• tag/thainer.py: Validates model paths before passing to pycrfsuite with version-specific instructions

Resource safety:

• tests/extra/testx_ulmfit.py: Replaced pickle.load(open(...)) with proper context manager

All changes maintain backward compatibility. Error messages use multiline format with actionable instructions:

RuntimeError(
    "ULMFiT model files not found. "
    "Please download the corpus first:\n"
    "  pythainlp.corpus.download('wiki_lm_lstm')\n"
    "  pythainlp.corpus.download('wiki_itos_lstm')"
)

Your checklist for this pull request

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

Original prompt

Review changes made in the following PRs and relevant codes or patterns:

#1232
#1231
#1230
#1228
#1225
#1220
#1214
#1213
#1212
#1211
#1202
#1190
#1189
#1186
#1182

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[coveralls]

coverage: 53.549% (-0.09%) from 53.636%
when pulling 31ffa48 on copilot/review-pr-changes
into 7cc69e9 on dev.

[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