chore: remove abi decoding from execute_program

[TomAFrench]

Related issue(s)

Resolves #

Description

Summary of changes

We only really make use of the decoded circuit return value which is returned from execute_program inside execute_cmd::run - in all other usages we just immediately drop it and just use the returned witness map.

This PR then moves the return value decoding up into execute_with_path and inlines solve_witness into execute_program.

Dependency additions / changes

Test additions / changes

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.
• This PR requires documentation updates when merged.

Additional context

[TomAFrench]

I've erred on the side of keeping the execute_program name rather than solve_witness as I expect this is more understandable to new-to-ZK devs.