fix: validate partial project payments against remaining amount

[sachinacharyaa]

What does this PR do?

This PR adds secure partial-payment support for project payments.

• Project payments are validated against the remaining project amount.
• Partial payments are explicitly allowed when the payment is greater than 0 and does not exceed the remaining amount.
• Bounty and grant payments retain the existing exact-match validation behavior.
• Prevents expectedAmount <= 0 from bypassing payment amount validation.
• Rejects zero/negative expected amounts unless an explicit partial-payment mode is enabled.

Where should the reviewer start?

Start with:

src/features/sponsor-dashboard/utils/paymentRPCValidation.ts

Then review the project payment verification flow, specifically how the remaining project amount is calculated and passed to validatePayment.

Finally, review the tests covering full, partial, zero, and excessive project payments.

How should this be manually tested?

1. Create a project with a payment amount greater than 0.
2. Make a payment for less than the remaining amount.
3. Verify the payment.
   • The payment should be accepted as a partial payment.
   • The remaining amount should be updated accordingly.
4. Make a payment for the exact remaining amount.
   • The payment should be accepted.
5. Attempt a payment of 0.
   • The payment should be rejected.
6. Attempt a payment greater than the remaining project amount.
   • The payment should be rejected.
7. Verify a bounty payment with an amount different from the expected amount.
   • It should still be rejected.
8. Verify a grant payment with an amount different from the expected amount.
   • It should still be rejected.

Any background context you want to provide?

Previously, project payment verification could rely on an expectedAmount of 0, which caused the amount validation to be skipped.

This change makes partial project payments explicit and validates them against the server-derived remaining amount while preserving exact-match behavior for bounty and grant payments.

This also addresses Finding 3, where validatePayment could skip amount checks when expectedAmount <= 0.

What are the relevant issues?

Finding 3 — validatePayment skips amount checks when expectedAmount <= 0.

Related to the partial-payment findings for project payment verification.

Screenshots (if appropriate)

Not applicable — this is a backend/payment-validation change with no user-facing UI changes.

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📥 Commits

Reviewing files that changed from the base of the PR and between af39bd1 and 0dee910.

📒 Files selected for processing (2)

• src/features/sponsor-dashboard/utils/paymentRPCValidation.ts
• src/pages/api/sponsor-dashboard/listings/verify-external-payment.ts

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