test(typehints-gp[field-xref]) Module-scope lambda-default fixture

why: PR #36 review caught a function-scoped Sphinx build inside
`test_unsupported_default_falls_back_to_plain_text` — CLAUDE.md
requires every integration test's Sphinx build to live in a module-
or session-scoped fixture so the build cost is shared across runs.
The three sibling tests in the file already follow the rule; this
one was the outlier because it was the last test added and used a
one-off scenario.

what:
- Hoist the lambda fixture project's module source into a
  module-level `_LAMBDA_MODULE_SOURCE` constant alongside the
  existing `_DATA_ATTRIBUTE_MODULE_SOURCE` and `_CROSS_MODULE_*`
  constants.
- Add `lambda_default_html_result` as a
  `@pytest.fixture(scope="module")` wrapping
  `build_shared_sphinx_result`, mirroring the existing three
  fixtures in shape (parameter list, scenario construction,
  `purge_modules=("lambda_demo",)` argument).
- Convert `test_unsupported_default_falls_back_to_plain_text` to
  consume the fixture parameter and call `read_output` directly;
  the two assertions are unchanged so the test still pins the
  plain-text-fallback contract for unparseable lambda defaults.

Author: tony

tests/ext/typehints_gp/test_default_xref_integration.py

@@ -291,24 +291,26 @@ def test_cross_module_default_resolves_via_refspecific(
     assert 'class="xref py py-obj' in html
 
 
-@pytest.mark.integration
-def test_unsupported_default_falls_back_to_plain_text(
-    tmp_path_factory: pytest.TempPathFactory,
-) -> None:
-    """Unparseable defaults (lambdas) leave the span as plain text."""
-    module_source = textwrap.dedent(
-        """\
-        from __future__ import annotations
+_LAMBDA_MODULE_SOURCE = textwrap.dedent(
+    """\
+    from __future__ import annotations
 
 
-        def has_lambda_default(callback=lambda: 1) -> None:
-            \"\"\"Function with a lambda default.\"\"\"
-        """
-    )
+    def has_lambda_default(callback=lambda: 1) -> None:
+        \"\"\"Function with a lambda default.\"\"\"
+    """
+)
+
+
+@pytest.fixture(scope="module")
+def lambda_default_html_result(
+    tmp_path_factory: pytest.TempPathFactory,
+) -> SharedSphinxResult:
+    """Build a project with a lambda default exercising the plain-text fallback."""
     cache_root = tmp_path_factory.mktemp("default-xref-lambda-html")
     scenario = SphinxScenario(
         files=(
-            ScenarioFile("lambda_demo.py", module_source),
+            ScenarioFile("lambda_demo.py", _LAMBDA_MODULE_SOURCE),
             ScenarioFile(
                 "conf.py",
                 _CONF_PY.replace("__SCENARIO_SRCDIR__", SCENARIO_SRCDIR_TOKEN),
@@ -327,12 +329,19 @@ def has_lambda_default(callback=lambda: 1) -> None:
             ),
         ),
     )
-    result = build_shared_sphinx_result(
+    return build_shared_sphinx_result(
         cache_root,
         scenario,
         purge_modules=("lambda_demo",),
     )
-    html = read_output(result, "index.html")
+
+
+@pytest.mark.integration
+def test_unsupported_default_falls_back_to_plain_text(
+    lambda_default_html_result: SharedSphinxResult,
+) -> None:
+    """Unparseable defaults (lambdas) leave the span as plain text."""
+    html = read_output(lambda_default_html_result, "index.html")
 
     # The lambda text appears in the rendered output but not wrapped in an xref
     assert "lambda" in html