docs(conf): Add missing-reference handler to resolve py:data as :class: links

why: sphinx-autodoc-pytest-fixtures wraps every fixture return-type
identifier in a :class: cross-reference so that Sphinx and intersphinx
can resolve it.  Sphinx 8.x's Python domain only searches py:class and
py:exception entries for :class: references; TypeAliases land in the
inventory as py:data instead.  The result is a broken (unlinked) type
name in the fixture summary table for any TypeAlias return type.

The same handler was added to unihan-etl's conf.py for the same reason;
this brings libvcs in line with that approach.

what:
- Import typing as t at the top of conf.py
- Guard docutils / sphinx type imports under TYPE_CHECKING so mypy gets
  accurate types without a runtime dependency on types-docutils being
  present in every environment
- Add _on_missing_class_reference(): when a :class: reference cannot be
  resolved, fall back to searching all Python-domain objects so that
  py:data entries (TypeAliases) are found and linked
- Add setup(app) to wire the handler into Sphinx's missing-reference
  event

Author: tony

docs/conf.py

@@ -4,6 +4,13 @@
 
 import pathlib
 import sys
+import typing as t
+
+if t.TYPE_CHECKING:
+    from docutils import nodes
+    from sphinx import addnodes
+    from sphinx.application import Sphinx
+    from sphinx.environment import BuildEnvironment
 
 from gp_sphinx.config import make_linkcode_resolve, merge_sphinx_config
 
@@ -54,3 +61,33 @@
     rediraffe_redirects="redirects.txt",
 )
 globals().update(conf)
+
+
+def _on_missing_class_reference(
+    app: Sphinx,
+    env: BuildEnvironment,
+    node: addnodes.pending_xref,
+    contnode: nodes.TextElement,
+) -> nodes.reference | None:
+    if node.get("refdomain") != "py" or node.get("reftype") != "class":
+        return None
+    from sphinx.util.nodes import make_refnode
+
+    py_domain = env.get_domain("py")
+    target = node.get("reftarget", "")
+    matches = py_domain.find_obj(env, "", "", target, None, 1)
+    if not matches:
+        return None
+    _name, obj_entry = matches[0]
+    return make_refnode(
+        app.builder,
+        node.get("refdoc", ""),
+        obj_entry.docname,
+        obj_entry.node_id,
+        contnode,
+    )
+
+
+def setup(app: Sphinx) -> None:
+    """Connect missing-reference handler to resolve py:data as :class: links."""
+    app.connect("missing-reference", _on_missing_class_reference)