tests: hotplug API and virtual-device hotplug scenarios

[Youw]

Adds the test suite for the hotplug API (#764 / connection-callback), in two tiers, built on the virtual-device test harness from #815.

What is in here

• src/tests/test_hotplug_api.c — tier 1, HotplugAPI_<backend>: everything in the hotplug contract that is observable without a device event (argument validation, callback-handle properties, implicit hid_init(), hid_exit() teardown incl. a register→immediate-hid_exit stress loop, and a 2-thread register/deregister churn). Needs no device, no privileges: registered for all four backends and runs in the ordinary per-push CI matrix. Self-skips (77) when a backend reports hotplug as unsupported at runtime (e.g. libusb without LIBUSB_CAP_HAS_HOTPLUG).
• src/tests/test_hotplug.c — tier 2, Hotplug_<backend>: device-backed scenarios: async delivery, exactly-once ENUMERATE pass, callback-return deregistration (live + mid-pass), pass-before-live ordering, ARRIVED/LEFT payloads and path correlation, LEFT-without-ENUMERATE, VID/PID filtering, registration-order dispatch, the deregistration post-condition, and register/deregister from within a callback.
• test_virtual_device_unplug() / test_virtual_device_replug() — new provider calls that toggle device presence without destroying the provider context. Implemented for the uhid provider (UHID_DESTROY/UHID_CREATE2 on the same fd); the rawgadget/vhidmini2/IOHIDUserDevice providers return TEST_VDEV_UNAVAILABLE for now, so their Hotplug_* tests self-skip everywhere (they compile and are registered, ready for when toggling is implemented).

The tier-2 test follows a strict synchronization discipline (see src/tests/README.md): callbacks only deep-copy events into a lock-protected log; every expectation is a deadline-based predicate poll; absence assertions sit behind event barriers, not time windows; no retries.

Where each test runs

Test	Where it runs
HotplugAPI_hidraw, HotplugAPI_libusb	per-push, builds.yml ubuntu-cmake
HotplugAPI_winapi	per-push, builds.yml windows-cmake (MSVC, NMake, ClangCL, MinGW)
HotplugAPI_darwin	per-push, builds.yml macos-cmake
Hotplug_hidraw	per-push, builds.yml ubuntu-cmake (via modprobe uhid + sudo ctest)
Hotplug_libusb	builds + self-skips (incl. in the dedicated libusb-vhid-test job) until rawgadget unplug/replug exists
Hotplug_winapi	builds + self-skips (incl. in the dedicated win-vhid-test job) until driver-side presence toggling exists
Hotplug_darwin	builds + self-skips; local-only on an entitled Mac, like DeviceIO_darwin

⚠️ Expected to be red until the backend PRs land

The behavioral expectations encode the async-ENUMERATE contract, i.e. they require #822, #823, #824 and #825. On connection-callback as-is, tier 1 fails (e.g. *callback_handle is not zeroed on a failed registration yet), so this PR's CI will be red until those four PRs land — that is expected and is the reason this PR is a draft.

Validation ahead of that: an integration branch (hotplug-integration = connection-callback + all four backend PRs + these tests) runs the same CI; there, HotplugAPI_hidraw/HotplugAPI_libusb pass in a local WSL run of both backends and HotplugAPI_winapi passes locally on Windows/MSVC.

Relates to: #764

Assisted-by: claude-code:claude-fable-5