Closes #50785

Opening a .wav file (PCM 16-bit) caused Zed to freeze because the binary detection heuristic in analyze_byte_contentmisidentified it as UTF-16LE text. The heuristic determines UTF-16 encoding solely by checking whether null bytes are skewed toward even or odd positions. PCM 16-bit audio with small sample values produces bytes like [sample, 0x00], creating an alternating null pattern at odd positions that is indistinguishable from BOM-less UTF-16LE by position alone.

Why not just add more binary headers?

The initial approach (32d8bd7) was to add audio format signatures (RIFF, OGG, FLAC, MP3) to known binary header. While this solved the reported .wav case, any binary format containing small 16-bit values (audio, images, or arbitrary data) would still be misclassified. Adding headers is an endless game that cannot cover unknown or uncommon formats.

Changes

• Adds is_plausible_utf16_text as a secondary validation: when the null byte skew suggests UTF-16, decode the bytes and count code units that fall in C0/C1 control character ranges (U+0000–U+001F, U+007F–U+009F, excluding common whitespace) or form unpaired surrogates. Real UTF-16 text has near-zero such characters. I've set the threshold at 2% — note that this is an empirically derived value, not based on any formal standard.

Before fix

After fix

Before you mark this PR as ready for review, make sure that you have:

• Added a solid test coverage and/or screenshots from doing manual testing
• Done a self-review taking into account security and performance aspects
• Aligned any UI changes with the UI checklist

Release Notes:

• Fixed binary files (e.g. WAV) being misdetected as UTF-16 text, causing Zed to freeze.