fix(descriptor): clarify descriptor key expression errors

[Chibey-max]

Description

The descriptor command treated descriptor key expressions returned by
key derive as raw extended keys. As a result, users received a confusing
Base58 parsing error when passing the output from key derive to
descriptor.

This change detects descriptor key expressions before raw key parsing and
returns a clear error explaining the accepted input format and where full
descriptors can be used.

Fixes #303

Notes to the reviewers

• Existing raw xpub, xprv, and mnemonic handling is unchanged.
• The implementation uses the clearer-error solution described in the issue.
• Directly accepting descriptor key expressions would require deciding how an
  already-derived key expression maps to the command's external and internal
  descriptor pair.
• Added an integration test covering the complete
  key generate -> key derive -> descriptor workflow.
• Added a changelog entry under [Unreleased].

Checklists

All Submissions:

• I've signed my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy

New Features:

• I've added tests for the new behavior
• I've added documentation for the new behavior

Bugfixes:

• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed