Issue Type: Bug / Code Correctness
Version:

• Package: x402 middleware / SDK
• Affected versions: latest (main branch)
• Runtime: Node.js 18+

Description:
When handling incoming HTTP requests, the x402 middleware does not consistently validate malformed or partially missing payment-related headers. In some cases, invalid headers are passed downstream, resulting in unclear errors or unexpected behavior during payment verification.

Expected behavior:

• Incoming requests with malformed or incomplete x402 payment headers should be rejected early.
• Errors should clearly indicate which header is missing or invalid.
• No downstream payment processing should occur for invalid requests.

Proposed solution:

• Add strict validation for required x402 headers at the middleware entry point.
• Return a structured error response with a clear reason.
• Add unit tests for missing, malformed, and partially valid header scenarios.

Acceptance criteria:

• Invalid payment headers fail fast with descriptive errors.
• No unexpected runtime exceptions occur during header parsing.
• Tests cover common malformed header cases.

Additional information:

• Early validation improves security and developer debugging experience.
• Aligns with defensive request handling practices in payment middleware.