Improve error message / accept descriptor key expressions in descriptor command

[EliteCoder18]

Describe the enhancement

bdk-cli key derive returns a descriptor key expression (e.g. [fingerprint/path]xpub.../*), while bdk-cli descriptor expects a raw xpub/xprv. Passing the output of key derive directly to descriptor results in:

BIP32 error: base58 encoding error

The error doesn't explain that the input is a descriptor key expression rather than a raw extended key.

It would be helpful if descriptor either:-

• detected descriptor key expressions and emitted a more descriptive error or
• accepted descriptor key expressions directly.

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Use case

A natural workflow is:

key restore
    ↓
key derive
    ↓
descriptor

Since key derive produces a key intended for descriptors, it's reasonable to expect its output to be consumable by descriptor.

Instead, users encounter a low-level Base58 parsing error without any indication that the command expects a different representation.

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Additional context

Reproducer:

bdk-cli key derive --xprv "tprv..." --derivation_path "m/84h/1h/0h"

Output:

[bd2eb472/84'/1'/0']tprv.../*

Then:

bdk-cli descriptor --type wpkh "[bd2eb472/84'/1'/0']tprv.../*"

produces:

BIP32 error: base58 encoding error

The same descriptor key expression works correctly when wrapped in a descriptor and passed to:

bdk-cli wallet config --ext-descriptor "wpkh([bd2eb472/84'/1'/0']tprv.../*)"

which suggests the underlying parsing is already supported in that context.