fix(fspy): handle null ObjectName and correct absolute path logic on Windows

[Copilot]

Fixes Windows crash when running oxlint with --type-aware flag and cache invalidation failures. The crash occurred due to null pointer dereference in UNICODE_STRING handling. The cache failures occurred because the inverted absolute path logic prevented proper directory tracking.

Changes

Path resolution (convert.rs)

• Fixed inverted logic: relative paths now combine with RootDirectory, absolute paths pass through
• Changed absolute path check from \\ (UNC) to \ (device-relative paths)
• Handle null ObjectName by checking reference before dereferencing
• When ObjectName is empty, return RootDirectory path directly instead of calling PathAllocCombine

UNICODE_STRING handling (winapi_utils.rs)

• Interpret Length field as bytes (was incorrectly treated as u16 count)
• Handle null/empty Buffer by returning empty slice instead of creating invalid pointer

Symbol resolution (detour.rs, nt.rs)

• Add ntdll to dynamic symbol lookup (kernel32 → ntdll fallback)
• Add NtQueryDirectoryFileEx detour for directory enumeration on newer Windows

Test updates

• Create directories before testing readdir operations (Windows requires directory exists to distinguish READ vs READ_DIR access)
• Add PATH setup for oxlint to find tsgolint subprocess
• Add oxlint_type_aware test case

Example

Before (crash or wrong path):

let filename_str = unsafe { get_u16_str(&*(*self).ObjectName) }; // Crash if null
let is_absolute = /* UNC check */;
if is_absolute { /* combine - wrong! */ }

After (safe and correct):

let filename_str = if let Some(object_name) = unsafe { (*self).ObjectName.as_ref() } {
    unsafe { get_u16_str(object_name) }
} else {
    U16Str::from_slice(&[]) // Empty when null
};
let is_absolute = filename_slice.get(0) == Some(&b'\\'.into()) || /* drive */;
if !is_absolute { /* combine - correct! */ }
if filename_slice.is_empty() { return root_dir; } // Direct return for empty

Original prompt

Fix the CI failure in #91

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