Export `PaymentParams` type from the package root

[0xNgoo]

Summary

The PaymentParams type used by buildPaymentXdr is reachable through the utils namespace but is not a named export from the package root.

Why this matters

A named root export makes it easier for callers to type the parameters they pass to the payment XDR builder.

Task

• Add the PaymentParams type as a named export from the package root entry point.
• Keep the existing utils.PaymentParams access path working.

Acceptance criteria

• PaymentParams is importable as a type from the package root.
• Existing exports remain intact.

Testing notes

• Add or update focused tests for the behavior in this issue.
• Run bun test before opening the pull request.
• Add or update a type or export test that proves the type is available from the root entry point.

Contributor notes

• Keep the change narrowly scoped to the behavior described above.
• Use Bun for commands and TypeScript for any code changes.
• Avoid adding new runtime dependencies for this issue.