Force UTF-8 output from yt-dlp to fix mojibake in redirected streams

[Aimeast]

Problem

On Windows, yt-dlp output captured through the library is garbled (mojibake).
For example the YouTube bot-check error message

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is reported as

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Root cause

yt-dlp is a Python program. When its stdout/stderr are redirected to a pipe,
Python picks their encoding from the system locale at process startup
(locale.getpreferredencoding() -> the ANSI code page, e.g. GBK/cp936 on
Chinese Windows) instead of always using UTF-8. The library always reads the
process output as UTF-8 (StandardOutputEncoding = Encoding.UTF8), so every
non-ASCII byte sequence was decoded into U+FFFD replacement characters.

Confirmed with yt-dlp.exe -v:

[debug] Encodings: locale cp936, fs utf-8, pref cp936, out gbk (No ANSI), error gbk (No ANSI), screen gbk (No ANSI)

Reading the pipes as GBK works on a GBK-locale machine but breaks on machines
with a different ANSI code page (e.g. cp1252) and is not future-proof once
yt-dlp ships builds that default to UTF-8 (Python 3.15+).

Fix

Append --encoding utf-8 to the yt-dlp arguments unless the caller already
set OptionSet.Encoding, which is respected and left untouched. This forces
yt-dlp to emit UTF-8 on both stdout and stderr regardless of the machine's
locale, matching the encoding the output is always read with.

The public API is unchanged - only the internal ConvertToArgs method in
YoutubeDLSharp/YoutubeDLProcess.cs was modified.

Verification

• dotnet build passes with 0 errors.
• Byte-level: without the fix, non-ASCII bytes in yt-dlp's output are GBK
  (B2 E2 CA D4 for 测试); with --encoding utf-8 they are UTF-8
  (E6 B5 8B E8 AF 95).
• End-to-end: ran YoutubeDLProcess.RunAsync with a real yt-dlp.exe
  (2026.07.04, PyInstaller build) on a URL whose error output contains
  non-ASCII text - before the fix every non-ASCII char became U+FFFD;
  after the fix the output decodes correctly with zero U+FFFD.
• Argument handling: default OptionSet produces --encoding utf-8;
  OptionSet.Encoding = "gbk" produces --encoding "gbk" (not overridden).