docs: add processing_failed to EmbedPaymentMethod ErrorCode in README

[detail-app]

Summary

The ErrorCode type in the checkout README was missing the 'processing_failed' variant
added to EmbedPaymentMethodErrorCode in source, and the onError description
incorrectly described the event as only firing on render-time failures.

What

• Added 'processing_failed' to the ErrorCode union in the events table.
• Broadened the onError prop description to cover payment-method flow failures,
  not just iframe render failures (token missing/expired).

Why

A developer implementing error handling against the documented type union would not
handle card declines or 3DS failures, since 'processing_failed' was absent from
the docs. Their onError callback would silently receive an undocumented code.

Introduced by commit d3e5319 (@sebastianekstrom, #11739)

How

Updated two rows in clients/packages/checkout/README.md to match the source
definition in clients/packages/checkout/src/payment-method.ts.

Checklist

• This PR addresses a single concern (one bug fix, one feature, one refactor)
• The diff is reasonably sized and easy to review
• New functionality is covered by tests
• Linting and type checking pass (uv run task lint && uv run task lint_types)
• No unrelated changes or drive-by fixes are included

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