intercept/buffer responses in flask middleware

[phdargen]

Description

Flask middleware sends response to client immediately when route handler is called, if settlement fails resource was obtained for free.

This is solved similar as in the x402-express middleware by intercepting response methods, buffering all calls and replaying after successful settlement.

Tests

Tested with flask server example + forcing settle_response.success = False in the flask middleware.

Client output before: { report: { temperature: 70, weather: 'sunny' } }

Client output after:

{
  x402Version: 1,
  accepts: [
    {
      scheme: 'exact',
      network: 'base-sepolia',
      maxAmountRequired: '1000',
      resource: 'http://localhost:4021/weather',
      description: '',
      mimeType: '',
      outputSchema: [Object],
      payTo: '0x1c47E9C085c2B7458F5b6C16cCBD65A65255a9f6',
      maxTimeoutSeconds: 60,
      asset: '0x036CbD53842c5426634e7929541eC2318f3dCF7e',
      extra: [Object]
    }
  ],
  error: 'Settle failed: Unknown error'
}

Checklist

• [x] I have formatted and linted my code
• [x] All new and existing tests pass
• [x] My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits

[cb-heimdall]

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