Fix the first line of UTF-16BE input swallowing the second line

In UTF-16BE a newline is 00 0A, so the initial read_until(b'\n') usually
already ends exactly at the end of line 1. The BE branch then always called
read_utf16_line, which read on to the next newline, merging lines 1 and 2.
Only keep reading when the first read did not end on a complete 00 0A.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VVn4SeAXPsUajXT2rtHQfo

CHANGELOG.md

@@ -25,6 +25,7 @@
 - Syntax highlighting for Python files using uv as script runner in shebang #3689 (@janlarres)
 
 ## Bugfixes
+- Fix the first line of UTF-16BE input swallowing the second line, which broke line numbers, `--line-range` and highlighting, see #XXXX (@cindykrafft)
 - Exit quietly with status 0 instead of panicking when stdout is closed early (broken pipe) on auxiliary output paths such as `--completion`, `--config-file`, `--diagnostic`, `--generate-config-file`, `--set-terminal-title` and `cache --build`/`--clear`, matching the main output path since #232, closes #4034, see #4035 (@Mathjk)
 - Track strikethrough (SGR 9 / 29) in the ANSI style tracker so it is re-emitted like bold, dim, italic, and underline, and is no longer dropped after the first highlighted token when displaying ANSI input with syntax highlighting, see #4015 (@mikehasa)
 - Allow boolean flags to be given more than once, so that a flag set in the config file can also be passed on the command line without erroring, see #3912 (@logarithmone1128)