fix(wallet_policy): apply template derivation in into_descriptor

[trevarj]

BIP-388 key information items are bare KEY expressions and the derivation
comes from the template's key placeholders, but the translator returned the
key information item verbatim, so wpkh(@0/**) materialized as wpkh(xpub...)
instead of wpkh(xpub.../<0;1>/*).

• take the derivation path and wildcard from the placeholder when
  materializing a descriptor key
• reject a key information item that is not an extended key, since there is
  nothing to append a derivation to

Fixes #1014

[trevarj]

Reminder to cherry pick into #1009 (comment) once merged

[apoelstra]

ACK 00b8e80; successfully ran local tests

[milah-247]

I reproduced this locally to verify the fix.

Checked out fix/wallet-policy-template-derivation, ran cargo build (clean build, no errors), then ran the exact reproduction case from #1014:

let mut policy = WalletPolicy::from_str("wpkh(@0/**)").unwrap();
let key = DescriptorPublicKey::from_str(
    "[6738736c/84'/0'/0']xpub6CRQzb8u9dmMcq5XAwwRn9gcoYCjndJkhKgD11WKzbVGd932UmrExWFxCAvRnDN3ez6ZujLmMvmLBaSWdfWVn75L83Qxu1qSX4fJNrJg2Gt",
).unwrap();
policy.set_key_info(&[key]).unwrap();
println!("{:#}", policy.into_descriptor().unwrap());

Output:
wpkh([6738736c/84'/0'/0']xpub6CRQzb8u9dmMcq5XAwwRn9gcoYCjndJkhKgD11WKzbVGd932UmrExWFxCAvRnDN3ez6ZujLmMvmLBaSWdfWVn75L83Qxu1qSX4fJNrJg2Gt/<0;1>/*)

The /<0;1>/* derivation path is now correctly present, matching the expected behavior in #1014. Confirms the fix works as described.