Metadata improvements and extra output for source verification

See also https://github.com/ethereum/sourcify/issues/1523#issuecomment-3163413317.

This is a tracking issue for several metadata-related improvements requested by Sourcify:
1. #14827 / #9332
1. #16152
1. #16150
1. #16123 (the part about metadata)
1. #16153 (optional)

Start with the linked Sourcify discussion and review issues #14827, #9332, #16152, #16150, #16123, and optional #16153. No files or tests are named; the tracking issue is done when the requested metadata improvements and extra source-verification output are addressed.

Written by the indexing model from the issue text.