fix restore_promises derivation-path mismatch on subset restores

[b-l-u-e]

This PR fixes a wallet restore issue where restored proofs could receive the wrong derivation_path when the mint returns only a subset of requested outputs (or returns them in a non-zero index range/order).
It also adds a regression test to ensure this mismatch cannot reoccur.

so here in restore_promises:

• secrets and rs were filtered using matching_indices
• but derivation_paths was not filtered before calling _construct_proofs(...)

1) Wallet fix

In cashu/wallet/wallet.py:

• filtered derivation_paths using the same matching_indices used for secrets/rs before _construct_proofs(...).

     secrets = [secrets[i] for i in matching_indices]
         rs = [rs[i] for i in matching_indices]
+        derivation_paths = [derivation_paths[i] for i in matching_indices]
         logger.debug(
             f"Restored {len(restored_promises)} promises. Constructing proofs."
         )

This restores 1:1 alignment among:

• restored_promises
• filtered secrets
• filtered rs
• filtered derivation_paths

2) Regression test

In tests/wallet/test_wallet_restore.py:

• added test_restore_promises_derivation_paths_subset
• test verifies each restored proof’s derivation_path matches the path expected for that proof’s secret
• catches subset/non-zero-index restore mismatch scenarios.

Test Plan

• poetry run pytest tests/wallet/test_wallet_restore.py::test_restore_promises_derivation_paths_subset -v

Note: While preparing this PR I hit a local pre-commit mypy error:
tests/conftest.py:119: Argument "seed" to "Ledger" has incompatible type "str | None"; expected "str"

not sure of the cause but maybe comes from here or rather maybe i am missing somethin???

git add tests/wallet/test_wallet_restore.py
git commit -s -m "test(wallet): add regression test for restore derivation-path mismatch"
trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check yaml...........................................(no files to check)Skipped
check for added large files..............................................Passed
check docstring is first.................................................Passed
check json...........................................(no files to check)Skipped
debug statements (python)................................................Passed
mixed line ending........................................................Passed
check for case conflicts.................................................Passed
ruff.....................................................................Passed
ruff-format..............................................................Passed
mypy.....................................................................Failed
- hook id: mypy
- exit code: 1

tests/conftest.py:119: error: Argument "seed" to "Ledger" has incompatible type "str | None"; expected "str"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

so i had to add an explicit guard in tests/conftest.py before constructing Ledger:

--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -114,6 +114,7 @@ async def ledger():
             Unit.usd: lightning_backend_usd,
         },
     }
+    assert settings.mint_private_key is not None
     ledger = Ledger(
         db=Database("mint", settings.mint_database),
         seed=settings.mint_private_key,

also i struggled with ruff-format for the cashu/wallet/wallet.py and ended up making changes in other part of the code file which is unrelated to the issue solved

[mercie-ux]

Ran this locally and verified that the regression test fails without filtering derivation_paths and passes with the fix. The alignment fix looks correct.