Expose PSBT input error index in FFI error types

[chavic]

PsbtInputsError index is dropped in FFI, making it hard to identify which input failed.

Todo

• Expose input index in FFI error type.
• Add tests to assert index propagation.
• Update error docs so integrators can surface precise diagnostics.

[spacebear21]

This seems like a non-issue as well because PsbtInputError is only used in InternalBuildSenderError which is opaque intentionally.