feat: implement Stripe PaymentIntent payment service (Closes #1)

[zhaocaixia888]

/claim #1

Replaced the stub payment service with real Stripe PaymentIntent creation.

• Stripe SDK integrated via STRIPE_SECRET_KEY
• Validates amount, currency
• Returns clientSecret and paymentId
• Stripe errors preserved with original message
• Unit tests + guarded live smoke test

Closes #1

[Adamchaua]

Monitoring this PR. For any approved payout: EVM 0x1ecab01075f3bdf1b56b7d849c8e28ef88943624; PayPal ckelvinkhanh32@gmail.com.

[mvmax-dev]

🔍 Detailed Technical Code Review & Correctness Bugs in PR #303

After reviewing the implementation in PR #303, there are critical bugs that prevent it from passing the repository's payment test suite:

1. Correctness Bug: Return Object Signature Mismatch

• The Bug: The test suite expects the return object of createPaymentIntent to conform to the exact signature:

  {
    clientSecret: string,
    paymentId: string,
    provider: "stripe"
  }

  PR #303 does not include the provider: "stripe" key in its return object. Instead, it returns { clientSecret, paymentId, amount, currency }, which fails the deep-equality assertions in the test suite.

2. Correctness Bug: Test-breaking Custom Error Wrapping

• The Bug: The unit test suite checks that when Stripe returns an error (e.g., card declined), the error is propagated such that err.message === "Card was declined" and the specialized Stripe properties are retained.
• Why it fails: PR #303 wraps the error in a custom PaymentError class and changes the error message to:
  "Stripe error (${err.type}): ${err.message}"
  This changes the message string entirely and fails the exact assertion err.message === "Card was declined".

3. Correctness Bug: Wrong Validation Error Messages

• The Bug: The test suite asserts the following exact validation messages:
  • Missing amount: Error("Amount is required")
  • Invalid/Non-integer amount: Error("Amount must be a positive integer")
    PR #303 throws custom errors with verbose messages:
  • Missing: PaymentError("amount is required and must be a positive integer (in cents).")
  • Invalid: PaymentError("amount must be a positive integer representing the smallest currency unit (e.g. cents).")
    This causes all validation tests to fail.

Due to these correctness issues and failing test conditions, this PR should be closed.