feat(ui-extensions-tester): add shopify.intercept mock and POS event data factories

[vctrchu]

Background

Stacked on #4641, which adds the POS intercept resolution target types. Extensions registering interceptors via shopify.intercept() had no way to unit-test their blocking logic: the tester exposed no intercept on the mock shopify global and no way to invoke a registered interceptor.

Solution

Mirrors the existing addEventListener/dispatch pair:

• shopify.intercept(name, interceptor) on the mock global — same semantics as the sandbox implementation: interceptor must be a function, event name must be a supported intercept event, one interceptor per event, returns an unregister function.
• extension.fireIntercept(name, data) — invokes the registered interceptor with {type, ...data} (the host payload shape) and returns its InterceptResult. Returns undefined when nothing is registered; throws if the interceptor returns a Promise, matching the host's synchronous-only rule.
• Event data factories — createCartValidationsEventData() and createPaymentValidationsEventData() build the payloads for fireIntercept() and double as shopify.resolution.event values on the resolution targets from feat(point-of-sale): add resolution target types for cart and payment validations #4641. createPosCart() is now exported (it was already referenced in the harness docs) and backs the cart event factory.

Both READMEs updated per the tester's documentation requirements.

🎩

• 12 new tests in pos-intercepts.test.ts covering registration rules, unregistration, fire semantics, sync enforcement, and factory defaults — 116 total passing
• yarn build, eslint, prettier — green
• Example test suites fail at vitest startup locally (ERR_REQUIRE_ESM, pre-existing on the base branch); relying on CI for those

Checklist

• I have 🎩'd these changes
• I have updated relevant documentation