fix(test): resolve the temp root so test_mirror_plan passes on macOS (dev is red)

[JustVugg]

dev is currently red on macOS only, in the test that arrived with #536:

AssertionError: PosixPath("/private/var/folders/.../same.safetensors")
            != PosixPath("/var/folders/.../same.safetensors")

Cause

On macOS /var is a symlink to /private/var. tempfile.TemporaryDirectory() hands back /var/folders/..., while mirror_plan.py resolves every path it is given — discover_shards (line 115) and create_plan (lines 197–199). The test then compared a resolved path returned by the tool against an unresolved one it had built itself.

Linux has no such symlink, so the two are identical there and the test passes — including in the run I did before merging #536, which is how this reached dev.

Fix

Resolve the temp root once in setUp, so every path derived from it is resolved and both sides of every assertion match on every platform. Test-only — mirror_plan.py is not touched.

Verification

Reproduced the macOS condition on Linux by creating a symlinked temp directory and running discover_shards through it:

  tool returns    : /tmp/real_xxx/model/a.safetensors
  old comparison  : /tmp/real_xxx_link/model/a.safetensors  -> FAILS
  new comparison  : /tmp/real_xxx/model/a.safetensors       -> PASSES

Plus the suite itself: 8 passed.