fix(mint): delete orphan blank outputs on melt change early-return

[gorrdy]

Summary

• _generate_change_promises deletes the wallet's blank NUT-08 outputs
  from the promises table after signing them, but the early-return
  branch (overpaid_fee <= 0 or no outputs supplied) skipped that
  cleanup. Those rows were left behind with c_ IS NULL and collided with
  later operations that re-derive the same B_ — most visibly NUT-13
  seed restore, which would surface as OutputsArePendingError.
• Apply the same delete_blinded_messages_melt_id cleanup in the
  early-return branch.
• Soften the overpaid_fee < 0 log from error to debug with a
  clearer message. In practice this is reachable whenever the LN backend
  reports a fee greater than the wallet-provided reserve (e.g. an LNbits
  backend skimming a service fee on top of the routing fee), so the
  original "this should not happen" wording is misleading and the
  error-level severity is noisy.
• Add a parametrized regression test covering both overpaid_fee == 0
  and overpaid_fee < 0 cases.

Test plan

• poetry run pytest tests/mint/test_mint_melt.py -k early_return -v passes both parametrize cases (overpaid_fee_zero,
  overpaid_fee_negative).
• Same tests fail on main (without the ledger fix) — orphan rows
  assertion trips.
• Full melt suite still green: poetry run pytest tests/mint/test_mint_melt.py.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.29%. Comparing base (aa2a3ff) to head (2620bef).
⚠️ Report is 72 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1016      +/-   ##
==========================================
+ Coverage   75.10%   75.29%   +0.19%     
==========================================
  Files         112      114       +2     
  Lines       12788    12970     +182     
==========================================
+ Hits         9604     9766     +162     
- Misses       3184     3204      +20     

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[a1denvalu3]

looks like the CI is failing

[KvngMikey]

@gorrdy , you've got this test failing:

`FAILED tests/mint/test_mint.py::test_generate_change_promises_zero_fee_deletes_all_blanks - assert 0 == 4

• where 0 = len([])`

[gorrdy]

Should be ok now.