[Bug]: msat-precision invoices fail to send (regular BOLT11 + LNURL pay/withdraw) #877

What happened?

When invoice amounts include millisatoshis that are not a whole satoshi, payment handling is incorrect.

Observed from repro and user reports:

• Regular BOLT11 invoices with valueMsat fail in app flows that should pay exact invoice amount.
• LNURL-pay / LNURL-withdraw flows are also affected for msat-precision limits.
• On Android specifically (LNURL case), the amount resolves to 0 for affected msat-precision flows.

This aligns with the original Slack report where invoice amount precision caused payment mismatch/failure.

Expected behavior

• Regular BOLT11 invoice payments with msat precision should succeed.
• LNURL-pay and LNURL-withdraw with msat-precision limits should preserve valid amount and execute.
• Android should not collapse msat-precision LNURL amount to 0.

Steps to Reproduce

1. Use regtest/local setup with active channel and spending balance.
2. Reproduce regular invoice case via E2E @send_3 flow (invoices created with lnd.addInvoice({ valueMsat })).
3. Try msat values like 222538, 222222, 500500.
4. Reproduce LNURL-pay/LNURL-withdraw msat cases (fixed min == max msat values).
5. Observe failure behavior (Android LNURL case shows amount as 0; regular invoice flow also fails for msat invoice handling).

Logs / Screenshots / Recordings

logs.zip

• Original report reference: Slack report

Bitkit Version

2.1.2

Device / OS

Android Emulator (sdk_gphone64_arm64), Android 13

Reproducibility

Always

Additional context

• E2E coverage for this bug class is in bitkit-e2e-tests PR #140 and should pass after fix.
• Coverage includes:
  • regular invoice msat cases in send.e2e.ts (@send_3)
  • LNURL pay/withdraw msat cases in lnurl.e2e.ts
• It would be good to add unit-level tests in amount parsing/payment amount calculation paths to prevent regressions.

Similar issues check

Potentially related Android issues reviewed:

• #790 (open) - Fail to send max / near max lightning payment
• #764 (closed) - LNURL-pay fails with decoding error in Bitkit
• #417 (open) - LNURL-pay comment not displayed on activity
• #418 (closed) - LNURL-withdraw ... minWithdrawable == maxWithdrawable ...