docs(split-payments): validate the allowed_combinations profile example

[vishkaty]

Problem

The only business-profile example for the split payments extension is annotated <!-- ucp:example skip -->, so CI never validates it, and it does not match the profile schema in three ways:

1. capabilities is an array, but ucp.json defines it as an object map keyed by reverse-domain name.
2. The capability entry omits the required schema URL (capability.json business_schema requires schema).
3. It pins "version": "2026-01-23", a release that does not contain this extension.

Validating the fragment against the profile schema fails on the capabilities type and then on the missing schema property; it passes once rewritten as a map with a schema URL.

Fix

Rewrite the example in the ucp.capabilities map form with spec, schema, extends, and {{ ucp_version }}, mirroring the buyer-consent profile example, and replace the skip annotation with a validated schema=profile def=business_schema annotation so CI covers it going forward.

extends: ["dev.ucp.shopping.checkout"] reflects split_payments.json's requires on checkout.

Verification

scripts/validate_examples.py full corpus goes from 343 to 344 passing (50 to 49 skipped): the example flips from skipped to validated. ucp-schema lint source/ unaffected.

[vishkaty]

A gentle note on this and its sibling docs and schema PRs (#659, #660, #661, #662, #669, #670, plus #625): all eight are rebased onto current main, conflict free, and validate_examples passes on each. They are independent of one another, so they can be reviewed in any order or batched, whatever is easiest. Happy to adjust any of them.

[vishkaty]

Following up on this docs and schema batch (#658, #659, #660, #661, #662, #669, #670, #625): all eight are still green on current main, conflict free, and independent, so any subset can be merged in any order. If the batch size is the awkward part, I am glad to fold the docs-only ones into a single PR to cut the review overhead, whatever is easiest on your side. No urgency at all, just flagging they are ready whenever the queue allows. Thanks @damaz91.

[vishkaty]

@jingyli you are a requested reviewer here, so flagging this quiet one. Small fix so the split payments profile example actually validates against the profile schema. Glad to adjust anything.

[dkoch74]

@knightlin-shopify I am supportive of fixing this example - could you please review?

[vishkaty]

Updated the spec URL in the example to the post-reorganization path, specification/payment/extensions/split-payments, matching what the other extension profiles carry on current main. The rest of the example is unchanged.