lang: Fix instruction return type generation with `declare_program!`

[acheroncrypto]

Problem

CPI client generation has typo with extra column (:) which causes compilation error when instruction return type is not ().

Summary of changes

• Remove the extra :
• Add a compilation test case with instruction return type

[vercel[bot]]

@acheroncrypto is attempting to deploy a commit to the coral-xyz Team on Vercel.

A member of the Team first needs to authorize it.