feat(contractor): migrate PaymentMethod to useContractorPaymentMethodForm hook

[serikjensen]

Summary

Migrates the Contractor PaymentMethod component from its monolithic inline-form pattern to the hook-based architecture (following /migrate-sdk-component-to-hooks). Single screen, single hook.

• Adds useContractorPaymentMethodForm — one headless hook for the combined payment-type + bank-account form. Uses a value-aware excludeFields function so the bank fields stay in requiredFieldsConfig (promotable) but are dropped from required validation when type === 'Check', mirroring the useContractorDetailsForm precedent.
• Rewrites PaymentMethod.tsx to BaseBoundaries + thin Root + SDKFormProvider, preserving the event surface (contractor/bankAccount/created, contractor/paymentMethod/updated, contractor/paymentMethod/done) and the masked-account-number behavior.
• Hardens pre-migration test coverage (PaymentMethod.test.tsx) and adds hook/schema unit tests.
• Exports the hook, schema, error codes, and fields from src/index.ts.
• Removes the now-unused PaymentTypeForm / BankAccountForm and the inline schema.

Notes

• Faithfully preserves current behavior, including the original "Direct Deposit always POSTs a bank account on submit" sequence. A follow-up is open for discussion: aligning the Direct Deposit path with gws-flows (drop the redundant PUT /payment_method since the bank-account POST already updates the payment method) and optionally gating the POST on actual field changes.

Test plan

• npm run test -- --run (full suite green)
• npm run lint:check (0 errors)
• Manual verification in the SDK dev app / Storybook

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