fix: fix finfo guesser in coroutines

[albertcht]

Problem

FileinfoMimeTypeGuesser cached finfo instances in a static class property($finfoCache). In a coroutine environment this is unsafe: a single finfo resource held in a static array is shared across all coroutines, which can lead to race conditions and undefined behavior when multiple coroutines concurrently call guessMimeType().

Solution

Replace the static array cache with Context::getOrSet(), which stores the finfo instance in Hypervel's coroutine context. Each coroutine gets its own isolated finfo instance, eliminating the shared-state problem while preserving the caching benefit within a single coroutine's lifetime.

The cache key is built from a fixed prefix plus the optional magic file path, matching the previous per-magic-file granularity:

// Before
private static $finfoCache = [];
$finfo = self::$finfoCache[$this->magicFile] ??= new finfo(FILEINFO_MIME_TYPE, $this->magicFile);

// After
private const FINFO_CACHE_KEY = '__support.finfo_mime_type_guesser.';
$finfo = Context::getOrSet(
    self::FINFO_CACHE_KEY . ($this->magicFile ?? ''),
    fn () => new finfo(FILEINFO_MIME_TYPE, $this->magicFile)
);