[artofbitcoin]

Description

Fixes #90.

Signer::get_key decided whether a key origin (fingerprint, path) covers a KeyRequest::Bip32 derivation with derivation.to_string().starts_with(&path.to_string()). That compares characters rather than path components, so:

• origin m/1 covered a request for m/10;
• origin 84h/1h/0h/1 covered 84h/1h/0h/10 and 84h/1h/0h/12/0;
• an unhardened step 0 covered its hardened counterpart 0h.

When both paths had the same length, skip(path.len()) left an empty suffix and the signer returned the key at the origin path for a request it did not cover.

The check now uses derivation.as_ref().starts_with(path.as_ref()) on the ChildNumber slices. A request equal to the origin path, or extending it, still matches, as covered by the existing get_key_bip32 cases.

Notes to the reviewers

• New test get_key_bip32_origin_is_not_a_string_prefix builds a descriptor with each origin and checks that the non-covered requests return Ok(None). It fails on master and passes with this change.
• alloc::string::ToString is no longer used in the non-test code, so its import moved to the test module.
• Checks run locally with Rust stable: cargo test (31 passed), cargo check --no-default-features --features miniscript/no-std, cargo clippy --all-targets --all-features -- -Dwarnings, cargo fmt --all -- --check.
• This change and its description were written with Claude Code (Claude Opus 5.5); I reviewed the diff and ran the checks above. The commit is signed by GitHub (web commit).

Changelog notice

Fixed: Signer no longer returns a key for a BIP32 request whose derivation only shares a string prefix with the key origin path.

Before submitting

• I followed the contribution guidelines
• This PR breaks the existing API

[noahjoeris]

Duplicate of #81