windows: fix XDG-related test failures on Windows (#11783)

## What

Two fixes for tests that fail on Windows due to Unix-specific
assumptions.

1. The "cache directory paths" test in xdg.zig hardcodes Unix paths like
`/Users/test/.cache` in expected values. The function under test uses
`std.fs.path.join` which produces native separators, so the expectations
need to match. Fixed by using `std.fs.path.join` for expected values
too, with a platform-appropriate mock home path.

2. Two shell integration tests for `setupXdgDataDirs` hardcode Unix path
separators (`:`) and Unix default paths (`/usr/local/share:/usr/share`).
These are not applicable on Windows where the delimiter is `;` and
`XDG_DATA_DIRS` is not a standard concept. Skipped on Windows with
`SkipZigTest`.


## Why skip instead of fix for the shell integration tests?

`setupXdgDataDirs` is used by fish, elvish, and nushell. On Windows,
`XDG_DATA_DIRS` is not standard. The equivalent would be `%ProgramData%`
(what Go's `adrg/xdg`, Python's `platformdirs`, and others map to).
Fixing this properly means adding a Windows-appropriate default, which
is a separate change. (How do you guys deal with these situations? Do
you create issues on the spot as reminders or do you wait for the
requirement to emerge by itself when the time comes?

Worth noting: the production code on line 664 of `shell_integration.zig`
hardcodes the fallback to `"/usr/local/share:/usr/share"` with `:`
separators, while `prependEnv` correctly uses `std.fs.path.delimiter`
(`;` on Windows). If a shell that uses this runs on Windows, you would
get mixed separators. Tracked separately.

## Verified

- `zig build test-lib-vt` passes on Windows (exit 0)
- No behavior change on Linux/macOS (xdg.zig fix produces same paths,
shell_integration skip only triggers on Windows)

## What I Learnt

- `std.fs.path.join` uses the native path separator, so tests that
hardcode `/` in expected paths will fail on Windows even if the
production code is correct. Better to use `path.join` in test
expectations too.
- The XDG Base Directory spec is Unix-only but cross-platform libraries
have converged on mappings. Ghostty maps to `%LOCALAPPDATA%` which
matches common conventions. The missing piece is `XDG_DATA_DIRS` which
has no Windows default and falls through to Unix paths.

src/os/xdg.zig

@@ -132,7 +132,7 @@ test {
 test "cache directory paths" {
     const testing = std.testing;
     const alloc = testing.allocator;
-    const mock_home = "/Users/test";
+    const mock_home = if (builtin.os.tag == .windows) "C:\\Users\\test" else "/Users/test";
 
     // Test when XDG_CACHE_HOME is not set
     {
@@ -140,7 +140,9 @@ test "cache directory paths" {
         {
             const cache_path = try cache(alloc, .{ .home = mock_home });
             defer alloc.free(cache_path);
-            try testing.expectEqualStrings("/Users/test/.cache", cache_path);
+            const expected = try std.fs.path.join(alloc, &.{ mock_home, ".cache" });
+            defer alloc.free(expected);
+            try testing.expectEqualStrings(expected, cache_path);
         }
 
         // Test with subdir
@@ -150,7 +152,9 @@ test "cache directory paths" {
                 .subdir = "ghostty",
             });
             defer alloc.free(cache_path);
-            try testing.expectEqualStrings("/Users/test/.cache/ghostty", cache_path);
+            const expected = try std.fs.path.join(alloc, &.{ mock_home, ".cache", "ghostty" });
+            defer alloc.free(expected);
+            try testing.expectEqualStrings(expected, cache_path);
         }
     }
 }

src/termio/shell_integration.zig

@@ -670,6 +670,8 @@ fn setupXdgDataDirs(
 }
 
 test "xdg: empty XDG_DATA_DIRS" {
+    if (builtin.os.tag == .windows) return error.SkipZigTest;
+
     const testing = std.testing;
 
     var arena = ArenaAllocator.init(testing.allocator);
@@ -696,6 +698,8 @@ test "xdg: empty XDG_DATA_DIRS" {
 }
 
 test "xdg: existing XDG_DATA_DIRS" {
+    if (builtin.os.tag == .windows) return error.SkipZigTest;
+
     const testing = std.testing;
 
     var arena = ArenaAllocator.init(testing.allocator);