docs(pytest-plugin[types]): Register CreateRepoFn and CreateRepoPostInitFn for cross-referencing

why: The fixture summary table rendered return types like `CreateRepoFn`
as plain text instead of hyperlinks. sphinx_autodoc_pytest_fixtures
emits :class:`~libvcs.pytest_plugin.CreateRepoFn` for every fixture
returning that type, but Sphinx silently degrades to plain text when
the name is absent from the Python domain's object index — which it
was, since the Protocol classes had no autoclass directive anywhere.
what:
- Add a "Types" section to docs/api/pytest-plugin.md with autoclass
  directives for CreateRepoFn and CreateRepoPostInitFn
- Use :special-members: __call__ to expose the callback contract
- Use :exclude-members: __init__, _abc_impl, _is_protocol to suppress
  Protocol internals surfaced by gp-sphinx's global autodoc defaults
  (private-members: True, autodoc_class_signature: "separated")

Author: tony

docs/api/pytest-plugin.md

@@ -41,3 +41,15 @@ def setup(
     pass
 ```
 :::
+
+## Types
+
+```{eval-rst}
+.. autoclass:: libvcs.pytest_plugin.CreateRepoFn
+   :special-members: __call__
+   :exclude-members: __init__, _abc_impl, _is_protocol
+
+.. autoclass:: libvcs.pytest_plugin.CreateRepoPostInitFn
+   :special-members: __call__
+   :exclude-members: __init__, _abc_impl, _is_protocol
+```