test(launcher, headless, async_api): add 32 Phase 10 gap-coverage tests

Final sweep adds unit tests for the modules left at 0% direct coverage
after Phases 1-9:

- launcher._tz_env: 7 tests covering the IANA -> POSIX mapping
  including the Phoenix / Honolulu no-DST regression cases
- launcher._humanize_max_seconds, _default_context_kwargs: 11 tests
  on the constructor-side helpers (no browser launch)
- _headless.make_virtual_display dispatcher + _WindowsVirtualDesktop
  init/teardown: 8 tests (Linux dispatch branch covered without
  spawning Xvfb, since __init__ does no I/O)
- async_api.InvisiblePlaywright constructor parity with sync: 8 tests
  guarding against drift between the two APIs

Suite: 230 -> 264 passing. Pyramid stays clean: 243 unit / 12
integration / 9 e2e.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_headless.py

@@ -0,0 +1,95 @@
+"""Unit tests for the ``_headless`` virtual-display dispatcher.
+
+The dispatcher (``make_virtual_display``) is the only piece of
+``_headless`` we can exercise as a unit test on a single platform:
+``_WindowsVirtualDesktop`` actually creates a Win32 desktop on
+construction's later ``start()`` call, and ``_LinuxVirtualDisplay`` calls
+``Xvfb`` — both belong in integration/E2E coverage. The dispatcher's
+job is pure platform routing, which we patch via ``monkeypatch``.
+
+Per scope: Windows-specific + platform-agnostic only. We still cover
+the Linux dispatch branch because instantiating ``_LinuxVirtualDisplay``
+does no I/O — Xvfb is only spawned in ``start()``, which we never call.
+"""
+from __future__ import annotations
+
+import pytest
+
+import invisible_playwright._headless as headless
+from invisible_playwright._headless import (
+    _LinuxVirtualDisplay,
+    _WindowsVirtualDesktop,
+    make_virtual_display,
+)
+
+
+@pytest.mark.unit
+def test_make_virtual_display_returns_windows_desktop_on_win32(monkeypatch):
+    monkeypatch.setattr(headless.sys, "platform", "win32")
+    vd = make_virtual_display()
+    assert isinstance(vd, _WindowsVirtualDesktop)
+
+
+@pytest.mark.unit
+def test_make_virtual_display_returns_linux_xvfb_on_linux(monkeypatch):
+    """``__init__`` of ``_LinuxVirtualDisplay`` does no I/O — only ``start()``
+    spawns Xvfb. Exercising the dispatcher here is safe on any host."""
+    monkeypatch.setattr(headless.sys, "platform", "linux")
+    vd = make_virtual_display()
+    assert isinstance(vd, _LinuxVirtualDisplay)
+
+
+@pytest.mark.unit
+def test_make_virtual_display_accepts_linux_variants(monkeypatch):
+    """``sys.platform`` can be ``linux2`` on older Pythons / WSL builds.
+    The dispatcher uses ``startswith("linux")`` to accept all variants."""
+    monkeypatch.setattr(headless.sys, "platform", "linux2")
+    assert isinstance(make_virtual_display(), _LinuxVirtualDisplay)
+
+
+@pytest.mark.unit
+def test_make_virtual_display_raises_on_darwin(monkeypatch):
+    """macOS is unsupported — the dispatcher must raise with a clear
+    message rather than returning a no-op shim. ``InvisiblePlaywright``
+    relies on this to bail before launching Firefox on a system where
+    the patched binary doesn't exist."""
+    monkeypatch.setattr(headless.sys, "platform", "darwin")
+    with pytest.raises(RuntimeError, match="Windows and Linux only"):
+        make_virtual_display()
+
+
+@pytest.mark.unit
+def test_make_virtual_display_raises_on_unsupported_platform(monkeypatch):
+    monkeypatch.setattr(headless.sys, "platform", "freebsd14")
+    with pytest.raises(RuntimeError, match="Windows and Linux only"):
+        make_virtual_display()
+
+
+@pytest.mark.unit
+def test_make_virtual_display_error_mentions_offending_platform(monkeypatch):
+    """Error message should include the actual ``sys.platform`` so the
+    user can diagnose why their CI / weird container is being rejected."""
+    monkeypatch.setattr(headless.sys, "platform", "sunos5")
+    with pytest.raises(RuntimeError, match="sunos5"):
+        make_virtual_display()
+
+
+@pytest.mark.unit
+def test_windows_desktop_initial_state_is_clean():
+    """Construction must not allocate Win32 resources — only ``start()``
+    does. Allows users to instantiate ``InvisiblePlaywright`` without
+    pywin32 installed; the import error fires lazily when ``start()`` runs."""
+    vd = _WindowsVirtualDesktop()
+    assert vd._desktop is None
+    assert vd._original_handle == 0
+
+
+@pytest.mark.unit
+def test_windows_desktop_stop_is_idempotent_without_start():
+    """``stop()`` after never calling ``start()`` must be a no-op, so
+    ``__exit__`` from a failed launch can call it unconditionally."""
+    vd = _WindowsVirtualDesktop()
+    vd.stop()
+    vd.stop()
+    assert vd._desktop is None
+    assert vd._original_handle == 0