fix: clamp n to len(pwcs) in wcswidth/wcstwidth to avoid IndexError

[gaoflow]

Bug

`wcswidth()` and `wcstwidth()` raise `IndexError` when the `n` parameter is larger than the length of the string.

```python

from wcwidth import wcswidth
wcswidth('hello', 10)
IndexError: string index out of range
```

Root cause: The `end` variable is set directly to `n`:

```python
end = len(pwcs) if n is None else n
```

When `n > len(pwcs)`, the `while idx < end` loop tries to access `pwcs[idx]` past the end of the string.

Fix

Clamp `end` to `len(pwcs)`:

```python
end = len(pwcs) if n is None else min(n, len(pwcs))
```

This is the natural Python semantics for sized strings: `n` larger than the string length is equivalent to measuring the whole string. The same fix applies to `wcstwidth()`.

Test

Added `test_wcswidth_n_exceeds_length` covering ASCII, wide characters, and ZWJ clusters.

```
1316 passed, 10 skipped
```

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.79%. Comparing base (9df7261) to head (49c5e74).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #227      +/-   ##
===========================================
- Coverage   100.00%   99.79%   -0.21%     
===========================================
  Files           27       27              
  Lines         1924     1967      +43     
  Branches       456      462       +6     
===========================================
+ Hits          1924     1963      +39     
- Misses           0        2       +2     
- Partials         0        2       +2     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 108 untouched benchmarks

---

_{Comparing gaoflow:fix-wcswidth-n-exceeds-length (49c5e74) with master (d1c99fe)}

[gaoflow]

Pushed ab680a8 to address the red checks:

• applied the docformatter change requested for the new docstring
• added wcstwidth(..., n > len(...)) coverage for the same overflow case
• added narrow_wider terminal override coverage so the _wcswidth.py / _width.py missing lines are exercised

Local verification:

• PYTHONPATH=$PWD uv run --python python3.11 --with-requirements requirements-tests39.txt python -m pytest tests/test_core.py::test_wcswidth_n_exceeds_length tests/test_term_overrides.py::test_narrow_wider_width -q
• uvx --python python3.11 --from docformatter==1.7.7 docformatter --check --diff --recursive --pre-summary-newline --wrap-summaries=100 --wrap-descriptions=100 wcwidth/ bin tests/
• full local Python 3.11 test suite with coverage: 1318 passed, 10 skipped; _wcswidth.py and _width.py both report 100% in that run.

[jquast]

Thanks, this will be in next release #228

Not sure if you caught this bug from integration, or from linting, but in general this argument is not recommended to be used, #208 (comment) but I have since removed this recommendation