Fix error serialization in payment middleware responses

[madisoncarter1234]

Summary

Fixes a bug where Error objects were being serialized to JSON in payment middleware error responses, resulting in empty objects {} instead of meaningful error messages.

Problem

When JavaScript Error objects are passed to JSON.stringify(), they serialize to empty objects:

JSON.stringify({error: new Error('test')}) // {"error":{}}

This affected all error responses in the x402 payment middleware across Express, Next.js, and Hono implementations. Clients would receive responses like:

{"x402Version": 1, "error": {}, "accepts": [...]}

Making debugging impossible.

Solution

Changed error handling in catch blocks to extract the error message:

// Before
error: error instanceof Error ? error : "Fallback message"

// After  
error: error instanceof Error ? error.message : "Fallback message"

Changes

• x402-express (src/index.ts): Fixed 3 catch blocks at lines 248, 282, 335
• x402-next (src/index.ts): Fixed 2 catch blocks at lines 275, 343
• x402-hono (src/index.ts): Fixed 2 catch blocks at lines 248, 312
• Updated all test files to expect string error messages instead of Error objects

Testing

All tests pass:

• x402-express: 30 tests passed
• x402-next: 35 tests passed
• x402-hono: 36 tests passed

Clients now receive proper error messages:

{"x402Version": 1, "error": "Invalid payment", "accepts": [...]}

[cb-heimdall]

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