fix(send): reject eligible inputs without private keys

[reez]

Description

Looking through derive_sp I noticed it can continue when the private key for an eligible input is missing as long as it finds a key for another input. That gives the sender an incomplete input key sum which won’t match what the receiver calculates, so this PR returns KeyError instead. (Inputs that aren’t eligible for silent payments are still skipped)

Notes to the reviewers

Added coverage for a missing key among multiple eligible inputs and for skipping an ineligible input.

Changelog notice

Return an error when an eligible silent payment input key is missing.

Checklists

All Submissions:

• I've signed all my commits
• I followed the conventional commit guidelines
• I ran just p (fmt, clippy and test) before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[nymius]

cACK 913b997

Thanks for reviewing the code! Good find.

[nymius]

ACK 91698ff