Fix symlink spec on Windows machines that can create symlinks - #2510

createSymlink falls back to copying when fs.symlink throws, and the spec encoded that fallback as a platform rule. Windows does create symlinks with Developer Mode on, so on those machines createSymlink takes the symlink path and two assertions fail while nothing is broken. CI never caught it because the runners have no symlink privilege, which is also why the platform check looked right.

The node:fs/promises mock in this file already wraps symlink, so it now records whether the real call went through and the assertions follow that. Six lines, no probing, no new helper.

npx vitest run src/utils/symlink.spec.ts --root packages/hub

Before, on main (Windows 11, Developer Mode on):

 × should create a symlink
   → expected true to be false
 × should work when symlinking twice
   → expected true to be false
 ✓ should work when symlink doesn't work (windows)

 Tests  2 failed | 1 passed (3)

After:

 ✓ should create a symlink
 ✓ should work when symlinking twice
 ✓ should work when symlink doesn't work (windows)

 Tests  3 passed (3)

One thing this gives up: the old assertion demanded true unconditionally on Linux, so it would have failed if fs.symlink ever stopped working there. Keeping that guard would mean putting a process.platform branch back in, and the copy path is already covered by the third test.

Test file only. No source change, no dependency.