Return error from at_derivation_index when descriptor has no wildcard

[febyeji]

Closes #829.

• Previously, calling at_derivation_index() on a descriptor without wildcards would silently ignore the index and return the descriptor unchanged. This was error-prone because callers would expect different indices to produce different addresses.
• Callers that need to convert a non-wildcard descriptor to a definite one should use DefiniteDescriptorKey::new() on individual keys, or check has_wildcard() before calling at_derivation_index().
• Add edge case tests for at_derivation_index wildcard check

[trevarj]

But what should the workflow be if you just want to convert a DescriptorPublicKey descriptor to a DefiniteDescriptorKey one, not touching wildcards if they don't exist?

I'm ok with this as long as we address what Andrew said in the linked issue. I also would like a way to convert from Descriptor<DescriptorPublicKey> into Descriptor<DefiniteDescriptorKey> since I am currently using at_derivation_index as an agnostic translation.

Maybe it can be solved with just adding a helper function into_definite() or something.

[febyeji]

Thanks for the review! Extracted a public into_definite() method which resolves the duplication and also addresses Andrew's question in the issue. find_derivation_index_for_spk now just calls self.into_definite()?.derived_descriptor(secp).