test(pt_expt): restore sys.modules snapshot in plugin entry-point test (deepmodeling#5728)

## Summary

`source/tests/pt_expt/test_plugin.py` (from deepmodeling#5559) pops `deepmd.pt_expt`
from `sys.modules` without restoring it, leaving the package's cached
submodules bound to a dead parent. Any later import of a cached
submodule (e.g. `deepmd.pt_expt.infer.deep_eval`) re-creates a BARE
parent package whose `utils`/`infer` attributes are never rebound, and
`mock.patch("deepmd.pt_expt.utils...")` in
`test_deep_eval_serialize_api.py` then fails with `AttributeError:
module 'deepmd.pt_expt' has no attribute 'utils'` under py3.10's mock
target resolution (py3.13 tolerates it).

The failure is shard-order dependent: it needs (1) something to import
`deepmd.pt_expt.infer.deep_eval` before `test_plugin` runs, and (2) the
serialize-API test to run after — so it appears/disappears as PRs add
test files and reshuffle the duration-based shards. It currently fails
`Test Python (4, 3.10)` on deepmodeling#5714 and `Test Python (8, 3.10)` on deepmodeling#5717
while master stays green by ordering luck.

## Fix

Snapshot the whole `deepmd.pt_expt.*` module tree before the re-import
and restore it (including the `deepmd` parent-package attribute binding)
in the `finally`.

Verified: fixed test passes together with the serialize-API tests; an
inline emulation of the worst-case ordering (child cached →
pop/reimport/pop → `mock.patch` target resolution) resolves cleanly.

Same one-file fix is cherry-picked on deepmodeling#5714 (as 6422007) and deepmodeling#5717;
whichever lands first, the others resolve trivially.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Tests**
* Improved import handling in the plugin loading test to better isolate
module state between test runs.
* Reduced the chance of leftover cached imports affecting later tests in
the same session.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Han Wang <wang_han@iapcm.ac.cn>

Author: wanghan-iapcm

source/tests/pt_expt/test_plugin.py

@@ -50,8 +50,14 @@ def fake_entry_points(*, group=None):
         ]:
             sys.modules.pop(k, None)
         sys.modules.update(saved)
-        if deepmd_pkg is not None and "deepmd.pt_expt" in saved:
-            deepmd_pkg.pt_expt = saved["deepmd.pt_expt"]
+        if deepmd_pkg is not None:
+            if "deepmd.pt_expt" in saved:
+                deepmd_pkg.pt_expt = saved["deepmd.pt_expt"]
+            elif hasattr(deepmd_pkg, "pt_expt"):
+                # deepmd.pt_expt was not imported before this test; the fresh
+                # import bound it onto the parent package, so drop that stale
+                # attribute to leave the parent exactly as we found it.
+                delattr(deepmd_pkg, "pt_expt")
 
     assert groups == ["deepmd.pt_expt"]
     assert calls == ["load"]