Closes #1576

Adds an optional beforePayment (TypeScript) / before_payment (Python) field to the x402 action provider config. The provider builds its x402 client internally, so users could not reach the SDK's existing hook; this passes the function to x402Client.onBeforePaymentCreation / x402ClientSync.on_before_payment_creation. Returning { abort: true, reason } (TS) or an AbortResult (Python) blocks the payment before it is signed. Uses: screening payTo against a sanctions list, per-seller spend policies, audit logging.

x402ActionProvider({
  maxPaymentUsdc: 1,
  beforePayment: async ({ selectedRequirements }) => {
    if (blocked(selectedRequirements.payTo)) return { abort: true, reason: "payTo not allowed" };
  },
});

• No new dependencies; no behaviour change when the field is not set.
• The TS type comes from the SDK: Parameters<x402Client["onBeforePaymentCreation"]>[0].
• Tests (TS and Python): the hook is registered when configured and not otherwise.
• Changeset and changelog fragment included.

Checked locally: jest src/action-providers/x402 (23 passed), tsc --noEmit, eslint, prettier; pytest tests/action_providers/x402 (12 passed), ruff check and format.

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