fix(hx-preload): build preloads with hx-headers and htmx:config:request

[mhalikosen]

Description

A preload is built with createRequestContext and a bare fetch, so it skips the header work a triggered request does: hx-headers never runs, htmx:config:request never fires, and the HX-Preloaded: true header the docs promise is never sent. The click then reuses that response, although its own request would have carried different headers.

The preload listener now follows __handleTriggerEvent:

• hx-vals receives {ctx} and publishes ctx.vals, hx-headers applies, and form, submitter and body are assigned to ctx.request.
• HX-Preloaded is set, then htmx:config:request fires; cancelling it drops the preload. The click fires the event again, and the header lets a listener tell the two apart.
• HX-Request-Type is computed after the event, as core does.
• The body is folded into the query string after the event, so a listener's changes reach the URL, and then cleared, since fetch rejects a body on GET.

hx-prompt now cancels a preload. On four-dev the click prompts, then reuses a preload sent without HX-Prompt, so the answer never reaches the server; once preloads fire htmx:config:request, the element would also prompt twice.

The hx-preload and hx-prompt pages document both.

Corresponding issue: #4110

Testing

Five new tests, each failing on four-dev (372c6e3) and passing here:

• hx-preload: inherited hx-headers and HX-Preloaded reach the preload; header and body changes made in htmx:config:request reach the preload fetch, which carries no body; a listener cancels only the preload and the click is sent without HX-Preloaded; the click reuses a preload that carries hx-headers.
• hx-prompt: with hx-preload loaded, the prompt opens once, on the click, and the single request carries HX-Prompt.

npm test: 1785 passed, 0 failed. In Chrome 154, a boosted link under hx-headers:inherited and hx-config:inherited, with an htmx:config:request listener, sent one request, the preload, carrying all three headers and HX-Preloaded: true, and the click swapped it in. A preloaded button with hx-prompt prompted once and sent one request with the answer.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded