Increase Payload compatibility timeout default

[jmgasper]

Summary:

• raise the default Payload compatibility request timeout from 10 seconds to 30 seconds
• retain the 1,000-30,000 ms clamp and CONTENTFUL_PAYLOAD_REQUEST_TIMEOUT_MS override
• update timeout documentation and invalid-value fallback coverage

Validation:

• focused CMS tests: 44 passed
• npm run lint
• npm run verify:no-retired-cms-targets
• npm run build