docs(pytest-plugin[types]): Register GitCommitEnvVars for cross-referencing

why: The fixture summary table rendered by sphinx-autodoc-pytest-fixtures
emits :class: cross-references for every fixture's return type.  For
GitCommitEnvVars (now a py:data entry after gaining a docstring) to
resolve as a clickable link in that table, it must appear in the
autodata directive that populates the ## Types section — the same
pattern used for CreateRepoFn and CreateRepoPostInitFn.

what:
- Add .. autodata:: libvcs.pytest_plugin.GitCommitEnvVars at the top of
  the ## Types block so the type is both documented and reachable as an
  anchor from the fixture table link

Author: tony

docs/api/pytest-plugin.md

@@ -45,6 +45,8 @@ def setup(
 ## Types
 
 ```{eval-rst}
+.. autodata:: libvcs.pytest_plugin.GitCommitEnvVars
+
 .. autoclass:: libvcs.pytest_plugin.CreateRepoFn
    :special-members: __call__
    :exclude-members: __init__, _abc_impl, _is_protocol