fix(lightning): improve error message for unsupported LND partial self-payments

[Hardeezah]

Description

Refs #1112

Partial (MPP) self-payments fail because the manual routing path used by pay_partial_invoice (QueryRoutes + SendToRouteV2, and the REST equivalents) cannot forward allow_self_payment: neither
QueryRoutesRequest nor SendToRouteRequest exposes the flag, while only SendPaymentRequest does. A functional fix (routing partial payments through SendPaymentV2's manual dest/amt/payment_hash shard mode) is tracked separately in #1112.

This PR is a UX stopgap: it does not enable partial self-payments. When MINT_LND_ALLOW_SELF_PAYMENT is enabled and a partial-payment failure matches the known pathfinding-failure strings (target not found / no route), it appends a note explaining that the cause is an upstream LND limitation on the QueryRoutes path. This lets a mint operator distinguish an expected self-payment failure from an unexpected routing problem.

Changes Made

• cashu/lightning/lnd_grpc/lnd_grpc.py: in pay_partial_invoice, when the
  QueryRoutes/SendToRouteV2 call raises AioRpcError and the error
  message contains target not found or no route, append the note if the
  self-payment setting is enabled.
• cashu/lightning/lndrest.py: same handling for the route-fetch error
  branch of pay_partial_invoice.
• tests/lightning/test_lnd_partial_mocked.py: parametrized tests for both
  backends covering the note appended (setting enabled) and unchanged error
  (setting disabled).

Caveats

• String matching on target not found / no route is best-effort and LND-version-dependent; the tests assert the note only when these exact substrings appear.
• No successful-payment flow is changed: the note is appended only in the existing failure branches and only when the opt-in setting is enabled.

Testing

• make check clean (Ruff + mypy)
• make test local suite passes; new tests cover all modified lines

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.45%. Comparing base (0ae760f) to head (efd6ce1).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1113      +/-   ##
==========================================
+ Coverage   74.35%   74.45%   +0.09%     
==========================================
  Files         115      115              
  Lines       13155    13160       +5     
==========================================
+ Hits         9782     9798      +16     
+ Misses       3373     3362      -11     

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