Cannot symlink source directories with declare_directory

### Description of the bug:

Source directories cannot be symlinked by using an output declared using `ctx.actions.declare_directory`, whereas generated directories can. This means this cannot be used as a type of hint for whether to create a file or directory symlink on Windows (see https://github.com/bazelbuild/bazel/issues/26701).

This could be the same root cause as https://github.com/bazelbuild/bazel/issues/12954, which is another case where source directories are not considered to be directories.

### Which category does this issue belong to?

Core

### What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

I’ve created a reproducer here: [bazel-symlink-source-dir.tar.gz](https://github.com/user-attachments/files/21601992/bazel-symlink-source-dir.tar.gz)

Building `:generated_directory_symlink` works fine, but the similar `:source_directory_symlink` does not, giving the following error:

```
Error in symlink: symlink() with "target_file" file param requires that "output" be declared as a file (did you mean to use declare_file() instead of declare_directory()?)
```

### Which operating system are you running Bazel on?

Ubuntu

### What is the output of `bazel info release`?

release 8.3.1

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

N/a

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text
N/a
```

### If this is a regression, please try to identify the Bazel commit where the bug was introduced with bazelisk --bisect.

N/a

### Have you found anything relevant by searching the web?

No

### Any other information, logs, or outputs that you want to share?

No

Start with the attached bazel-symlink-source-dir.tar.gz reproducer and compare the working generated_directory_symlink target with source_directory_symlink using ctx.actions.declare_directory. Trace the handling of source directories and the symlink() target_file validation. Done means the source-directory case builds without the declare_file error and preserves the intended file-versus-directory symlink distinction on Windows.

Written by the indexing model from the issue text.