[6.x] Truncate full measure static cache files before writing

[joshuablum]

Right now, full measure static cache operations open cache files with fopen($path, 'c') and then fwrites the content. Mode c positions the pointer at the start of the file but does not truncate it. When a page is recached with content shorter than the version already on disk, the leftover bytes past the new content are still there producing a valid document followed by leftovers after the </html>.

Having background_recache enabled, saving an entry overwrites the file in place rather than deleting it, so any edit that shrinks a page leaves bits behind.

This fixes it by calling ftruncate($handle, 0) after acquiring the lock and before writing. Mode c keeps the pointer at byte 0 after truncation.

Closes #14742.