std.fs: End iteration on Linux/WASI during Iterator.next when hitting ENOENT

`getdents` on Linux can return `ENOENT` if the directory referred to by the fd is deleted during iteration. Returning null when this happens makes sense because:

- `ENOENT` is specific to the Linux implementation of `getdents`
- On other platforms like FreeBSD, `getdents` returns `0` in this scenario, which is functionally equivalent to the `.NOENT => return null` handling on Linux
- In all the usage sites of `Iterator.next` throughout the standard library, translating `ENOENT` returned from `next` as null was the best way to handle it, so the use-case for handling the exact `ENOENT` scenario specifically may not exist to a relevant extent

Previously, ENOENT being returned would trigger `os.unexpectedErrno`.

Closes #12211

lib/std/fs.zig

@@ -607,6 +607,7 @@ pub const IterableDir = struct {
                             .BADF => unreachable, // Dir is invalid or was opened without iteration ability
                             .FAULT => unreachable,
                             .NOTDIR => unreachable,
+                            .NOENT => return null, // The directory being iterated was deleted during iteration.
                             .INVAL => return error.Unexpected, // Linux may in some cases return EINVAL when reading /proc/$PID/net.
                             else => |err| return os.unexpectedErrno(err),
                         }
@@ -741,6 +742,7 @@ pub const IterableDir = struct {
                             .FAULT => unreachable,
                             .NOTDIR => unreachable,
                             .INVAL => unreachable,
+                            .NOENT => return null, // The directory being iterated was deleted during iteration.
                             .NOTCAPABLE => return error.AccessDenied,
                             else => |err| return os.unexpectedErrno(err),
                         }

lib/std/fs/test.zig

@@ -219,6 +219,30 @@ test "Dir.Iterator twice" {
     }
 }
 
+test "Dir.Iterator but dir is deleted during iteration" {
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+
+    // Create directory and setup an iterator for it
+    var iterable_subdir = try tmp.dir.makeOpenPathIterable("subdir", .{});
+    defer iterable_subdir.close();
+
+    var iterator = iterable_subdir.iterate();
+
+    // Create something to iterate over within the subdir
+    try tmp.dir.makePath("subdir/b");
+
+    // Then, before iterating, delete the directory that we're iterating.
+    // This is a contrived reproduction, but this could happen outside of the program, in another thread, etc.
+    // If we get an error while trying to delete, we can skip this test (this will happen on platforms
+    // like Windows which will give FileBusy if the directory is currently open for iteration).
+    tmp.dir.deleteTree("subdir") catch return error.SkipZigTest;
+
+    // Now, when we try to iterate, the next call should return null immediately.
+    const entry = try iterator.next();
+    try std.testing.expect(entry == null);
+}
+
 fn entryEql(lhs: IterableDir.Entry, rhs: IterableDir.Entry) bool {
     return mem.eql(u8, lhs.name, rhs.name) and lhs.kind == rhs.kind;
 }