fix: return descriptor template key type errors

[j-kon]

Description

Closes #1071.

The BIP44, BIP49, BIP84, and BIP86 descriptor constructors accepted descriptor key objects whose underlying single or multipath variants were handled with unreachable!(). This allowed generated binding callers to trigger a Rust panic after successfully parsing a WIF or other valid descriptor key.

This change makes every BIP template constructor return DescriptorError::InvalidKeyType for unsupported variants. The secret-key constructors now return Result, and their template build errors are propagated instead of unwrapped.

Breaking change

The four secret BIP template constructors are now throwable in generated bindings.

Validation

• cargo fmt --all -- --config format_code_in_doc_comments=true --check
• cargo clippy --all-targets --features "uniffi/bindgen-tests"
• CLASSPATH=./tests/jna/jna-5.14.0.jar cargo test --features uniffi/bindgen-tests

[reez]

Looking good thanks for taking this on. Please add the changelog: breaking label. Since the four secret constructors now throw/return a result I dont think this can merge into a near term non-breaking release, so could we also track or link it from the existing 4.0 changes issue #1036 so it’s queued up for the next breaking release? Otherwise the implementation and tests looking good to me.

[j-kon]

Thanks for the review. I linked #1072 from the 4.0 tracking issue #1036. I also attempted to add changelog: breaking, but GitHub rejected it because I do not have label permissions on this repository, so a maintainer will need to apply that label.

[Johnosezele]

Looking at the scope of things, wouldn't it be beneficial if we extract the shared require_xprv, require_xpub and from_template helpers so the validation lives in one place, I think there's already an existing debt that would keep compounding if someone later decides MultiXPrv should map to a different error than Single, or adds a fourth variant, they'd need to update eight sites... mssing one is how unreachable!() survived in the first place.

[j-kon]

@Johnosezele Addressed in b47a869: key-type validation now lives in shared require_xprv and require_xpub helpers, and template construction/error mapping lives in generic from_template. The eight exported constructors remain explicit for their UniFFI APIs. I also rebased onto current master and reran format, bindgen-feature clippy, and the full test suite.