fix: don't call winterm for OSC title codes when not converting (#407)

[patchwright]

Fixes #407.

Problem

In convert_osc, winterm.set_title() is called unconditionally when an OSC title sequence is seen. Every other winterm call is gated on self.convert (convert_ansi checks if self.convert:, get_win32_calls checks self.convert and winterm) — but this one isn't.

On non-Windows platforms winterm is None. When strip=True and convert=False (e.g. running without a PTY on Linux, where init() strips sequences but does not convert), writing a title sequence reaches winterm.set_title and crashes:

AnsiToWin32.write -> write_and_convert -> convert_osc -> winterm.set_title
AttributeError: 'NoneType' object has no attribute 'set_title'

(call path as traced in #407 by the reporter.)

Fix

Guard the call with self.convert and winterm, matching the pattern already used in get_win32_calls. The OSC sequence is still stripped from the output in all cases (that happens before the guard); only the win32 title call is now correctly skipped when not converting.

Tests

Added test_osc_codes_not_converted_when_stripping: with winterm patched to None and strip=True, convert=False, writing a title OSC sequence must not raise. It fails on main with the exact AttributeError above and passes with the fix. Existing test_osc_codes (the convert=True path) is unchanged. Full suite: 39 passed, 14 skipped.