tests(gp_sphinx_vite[ci]): pre-create node_modules in fake-vite roots

why: After 013884d added _ensure_node_modules() auto-install to the
builder-inited hook, six pre-existing tests started shelling out to
real `pnpm install --frozen-lockfile` because their fake vite roots
never had node_modules/. CI (no pnpm on PATH) failed with FileNotFoundError
across all 10 qa matrix jobs.

what:
- _write_fake_vite() gains with_node_modules: bool = True; default
  pre-creates node_modules/ so _ensure_node_modules short-circuits.
- test_on_builder_inited_runs_install_when_node_modules_missing now
  passes with_node_modules=False to keep its install-path assertion
  meaningful.
- Integration test test_sphinx_build_spawns_via_extension pre-creates
  node_modules/ in its fake-vite-root.
- Auto-install behaviour itself is unchanged; the three dedicated
  install-path tests (skips/runs/fails) still cover it.

Author: tony

tests/test_gp_sphinx_vite_hooks.py

@@ -40,9 +40,19 @@ class _FakeApp:
     config: _FakeConfig = dataclasses.field(default_factory=_FakeConfig)
 
 
-def _write_fake_vite(tmp_path: pathlib.Path, *, body: str) -> pathlib.Path:
-    """Write a fake-vite script + a stub package.json at ``tmp_path``."""
+def _write_fake_vite(
+    tmp_path: pathlib.Path, *, body: str, with_node_modules: bool = True
+) -> pathlib.Path:
+    """Write a fake-vite script + a stub package.json at ``tmp_path``.
+
+    Creates ``node_modules/`` by default so :func:`hooks._ensure_node_modules`
+    short-circuits the auto-install path. Tests that exercise the install
+    path explicitly pass ``with_node_modules=False`` and arrange their own
+    ``pnpm_install_command`` patch.
+    """
     (tmp_path / "package.json").write_text('{"name": "fake-vite-root"}\n')
+    if with_node_modules:
+        (tmp_path / "node_modules").mkdir(exist_ok=True)
     script = tmp_path / "fake_vite.py"
     script.write_text(textwrap.dedent(body))
     return script
@@ -339,6 +349,7 @@ def test_on_builder_inited_runs_install_when_node_modules_missing(
     )
     vite_script = _write_fake_vite(
         tmp_path,
+        with_node_modules=False,
         body="""\
         import time
         print("vite watching", flush=True)

tests/test_gp_sphinx_vite_integration.py

@@ -77,6 +77,9 @@ def test_sphinx_build_spawns_via_extension(tmp_path: pathlib.Path) -> None:
     fake_vite_dir = tmp_path / "fake-vite-root"
     fake_vite_dir.mkdir()
     (fake_vite_dir / "package.json").write_text('{"name": "fake-vite-integration"}\n')
+    # Pre-create node_modules/ so _ensure_node_modules short-circuits the
+    # auto-install path (CI runners don't have pnpm on PATH).
+    (fake_vite_dir / "node_modules").mkdir()
     fake_script = fake_vite_dir / "fake_vite.py"
     fake_script.write_text(
         textwrap.dedent(