docs: add comprehensive JSDoc to SplitClient methods

[kingsrichie79-cmyk]

Closes #698

Description:

Description

This PR resolves the issue regarding the lack of JSDoc comments on SplitClient methods in src/client.ts. By ensuring that all public methods are well-documented, we reduce the time new integrators need to spend reading the source code to understand how to use the SDK.

Changes Made

• Added detailed JSDoc blocks to every public method on SplitClient.
• Ensured every public method's JSDoc block includes @param, @returns, and @throws tags.
• Added practical @example blocks to the three most heavily utilized methods:
  • createInvoice
  • pay
  • getInvoice
• Explicitly marked all private/internal methods with the /** @internal */ tag to exclude them from external public-facing documentation.

Notes

• Documentation only: No method signatures or runtime behaviors were modified.
• All existing unit tests pass seamlessly as this is strictly a documentation update.

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