Mock rules JSON schema in tests and cancel state update on unmount

[gciotola]

Closes #1187

Problem

fetchJsonSchema was performing a real network request to
https://core.commercelayer.io/api/public/schemas/order_rules: no MSW handler
existed, and that URL was explicitly excluded from the onUnhandledRequest
warning in src/mocks/setup.ts. Depending on network latency the promise could
resolve after the test file had finished, running setOptionsConfig against
an already torn-down jsdom environment.

Changes

• src/mocks/data/core_schemas.ts (new): MSW handlers serving order_rules
  and price_rules from the local JSON files already present in
  RuleEngine/json_schema/, registered in src/mocks/handlers.ts. No more
  network calls in tests.
• src/mocks/setup.ts: removed the onUnhandledRequest bypass for that URL,
  so unmocked requests are reported again.
• RuleEngineComponent.tsx: the schema-parsing effect now returns a cleanup
  that sets a cancelled flag, preventing the state update after unmount. This
  is a general fix, relevant in production too on fast navigations.

Extra

If we want to keep testing against real API data, we can add the following test

it.concurrent("local order_rules schema matches remote", async () => {
  const remote = await fetch("https://core.commercelayer.io/api/public/schemas/order_rules").then(r => r.json())
  expect(remote).toEqual(orderRulesJsonSchema)
})

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