refactor(splitpayments): Improve LNURL payment reliability

[santyr]

Tested with LNBits v1.3.0-rc2

This refactor overhauls the get_lnurl_invoice function to make it more robust, reliable, and independent of the web API layer, ensuring that errors are logged correctly and payments are processed as expected.

Changes Made

Fixed ImportError for api_lnurlscan:

    The initial code attempted to import api_lnurlscan from lnbits.core.views.api, which was incorrect for the user's version of LNbits, causing an ImportError.

    This was corrected to import from lnbits.core.views.lnurl_api.

Fixed TypeError on Response Object:

    After fixing the import, the code produced a TypeError: 'LnurlPayResponse' object is not subscriptable.

    The api_lnurlscan function returns a Pydantic model, not a dictionary. The code was updated to use attribute access (data.callback) instead of dictionary key access (data["callback"]).

Replaced API call with Direct lnurl.handle to Fix Silent Failures:

    The final and most critical issue was that calling the api_lnurlscan endpoint suppressed underlying errors from the lnurl library, causing the payment process to fail silently without any logs.

    The implementation was changed to bypass the API endpoint entirely and use the lnurl.handle function directly. This provides more direct control and ensures that any exceptions are caught and logged within the extension's context, preventing silent failures.