fix(cookbook): load user-site pth hooks for runtime installs

Replay user-site .pth hooks when checking cookbook runtime dependencies so packages installed with --user are visible to dependency completion. Includes focused regression coverage.

routes/shell_routes.py

@@ -1063,8 +1063,19 @@ def setup_shell_routes() -> APIRouter:
         importlib.invalidate_caches()
         try:
             user_site = site.getusersitepackages()
-            if user_site and os.path.isdir(user_site) and user_site not in sys.path:
-                sys.path.append(user_site)
+            if user_site and os.path.isdir(user_site):
+                # Use addsitedir(), NOT a bare sys.path.append(). When a package
+                # is `pip install --user`'d at runtime (Cookbook → Install) the
+                # long-lived server process started before the user-site existed,
+                # so site never processed it — including its `.pth` hooks. On
+                # Python 3.12+ `distutils` is gone from stdlib and is only
+                # restored by setuptools' `distutils-precedence.pth`, which ships
+                # in user-site. basicsr (a realesrgan dep) does `import distutils`
+                # at import time, so a plain append left the package importable
+                # but `import distutils` failing → realesrgan probed as
+                # not-installed until a full process restart. addsitedir() replays
+                # the `.pth` files so the shim is active.
+                site.addsitedir(user_site)
         except Exception:
             pass
         if ssh_port and str(ssh_port).strip() not in ("", "22"):

tests/test_cookbook_dependency_completion_regression.py

@@ -106,4 +106,9 @@ def test_local_dependency_probe_refreshes_user_site_visibility():
 
     assert "importlib.invalidate_caches()" in source
     assert "user_site = site.getusersitepackages()" in source
-    assert "if user_site and os.path.isdir(user_site) and user_site not in sys.path:" in source
+    # addsitedir (not a bare sys.path.append) so user-site `.pth` hooks are
+    # replayed when a package is installed into an already-running process —
+    # otherwise setuptools' distutils shim never activates and basicsr-based
+    # deps (realesrgan) probe as not-installed until a restart. See #4810.
+    assert "if user_site and os.path.isdir(user_site):" in source
+    assert "site.addsitedir(user_site)" in source