fix(vidstack): correct slider video preview error handling

[TDaglis]

Related:

Fixes #1863

Related: #1090

Description:

Fixes two bugs in SliderVideo (TimeSlider.Video in React, <media-slider-video> as a custom element):

• Valid previews were always marked as errored and hidden. #hasError() checked isNull(error), which tests the signal function itself, and that is never null. As a result data-error was always set and hidden was true whenever the player could load, so the default theme (.vds-slider-video[data-hidden] { display: none }) never showed the preview. It now reads the value with isNull(error()), matching Thumbnail and Poster.
• Preview errors recursed in React. React's TimeSlider.Video attaches the component to the same <video> it listens to for error. #onError() redispatched error on that element and re-entered itself until the stack overflowed, calling the onError prop at every level. It now returns early for its own event (event.target === this, since Maverick's dispatch() sets target to the component). Native errors, whose target is the <video>, are still handled.

Adds slider-video.test.ts, covering a valid preview, an error with the video as the host (React) and inside the host (custom element), recovery after a source change, and seeking the preview to the pointer position.

Ready?

Yes.

Anything Else?

Reproduction with @vidstack/react@1.15.6: https://stackblitz.com/github/TDaglis/examples/tree/slider-video-repro/player/react/slider-video-repro?file=src/player.tsx

Checked in a browser against the published @vidstack/react@1.15.6 and against this branch (React sandbox):

	1.15.6	This PR
Valid preview	data-error, data-hidden, display: none	Visible, no error flags
Missing preview	onError × 44, then RangeError: Maximum call stack size exceeded	onError × 1, hidden

Vitest resolves Maverick's server build, where dispatch() is a no-op, so the test re-creates the browser dispatch (including the target override) on a jsdom element. It caps recursion so the pre-fix run fails without overflowing the runner.

pnpm format:check, pnpm typecheck, pnpm build and pnpm test pass locally (Node 22, pnpm 8.7.0).

Review Process:

• Open the reproduction: on 1.15.6 the valid preview reports data-error: true and display: none, and the missing preview counts dozens of onError calls.
• pnpm -F vidstack exec vitest --run src/components/ui/sliders/slider-video.test.ts passes; with slider-video.ts reverted, 3 of the 5 tests fail.
• Optional: in the React sandbox, render TimeSlider.Video with the default theme and one valid and one missing src. The valid preview is visible on hover, and the missing one fires onError once.