Fix MultiByteToWideChar buffer sizing and allocation

[seer-by-sentry]

Fixes CLIENT-1FG. The issue was that: Buffer size calculation error in MultiByteToWideCharSingleLine corrupts IME composition string, causing TextInputServer read access violation.

• Correctly calculate the required wide character buffer size using MultiByteToWideChar before allocation.
• Allocate the buffer based on the calculated size (which includes the null terminator).
• Handle cases where MultiByteToWideChar fails or returns zero length.
• Ensure correct null termination of the resulting wide string at the final index.

This fix was generated by Seer in Sentry, triggered automatically. 👁️ Run ID: 8127706

Not quite right? Click here to continue debugging with Seer.