feat: fluent request builder + test faking/mocking

Summary

Two additive features for httx — no breaking changes to the existing options-object HttxClient API.

Closes #1759 (fluent builder) and #1760 (test faking).

#1759 — Fluent PendingRequest builder

import { http } from '@stacksjs/httx'

const res = await http()
  .withToken(token)
  .withHeaders({ 'X-Tenant': id })
  .acceptJson()
  .retry(3, 200)
  .timeout(5000)
  .post<CreatedOrder>('https://api.test/orders', body)
//    still returns Result<HttxResponse<CreatedOrder>, Error>

Builder methods: withHeaders / replaceHeaders / withToken / withBasicAuth / withUserAgent / accept / acceptJson / withQueryParameters / asJson / asForm / asMultipart / timeout / retry / baseUrl / withOptions, then terminal get/post/put/patch/delete/head<T>. Delegates to HttxClient.request() so retry, circuit breaker, and timing are reused.

#1760 — Test faking (Http::fake() equivalent)

httx.fake({ 'api.github.com/*': { status: 200, body: { login: 'octocat' } } })
const res = await httx.get('https://api.github.com/users/octocat')
httx.assertSent(req => req.method === 'GET')
httx.restoreFetch()

• URL glob patterns (optional METHOD prefix; protocol-less matching), function responders (req) => spec, and httx.sequence([...]) for ordered responses.
• Strict by default — an unmatched request throws HttxFakeUnmatchedError so a test can't silently hit the network; opt into { passthrough: true }.
• Assertions: assertSent / assertNotSent / assertSentCount / assertNothingSent + recorded().
• The intercept synthesizes a real Response, so the client's parse path is identical to a live request.

New httx facade

A process-wide singleton bundling the shared-client verbs, the http() builder (httx.request()), and the fake controls.

Test plan

• 28 new hermetic tests (test/fake.test.ts, test/pending-request.test.ts) — no network
• tsc --noEmit clean under --isolatedDeclarations
• pickier clean
• existing non-network suites (errors, utils) still green
• maintainer: confirm the httx facade naming + that strict-by-default faking is the desired default

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