Expose zero-only decimal CSV inference metadata

[osipovartem]

Summary

• Mark all-zero decimal fields in opt-in Arrow CSV inference with chunk-local metadata.
• This lets downstream streaming inferencers combine zero-only chunks with exact scale-38 decimals without falsely requiring an integer digit.
• The marker is not merge-stable under generic Field::try_merge; consumers must inspect it before merging fields.

Validation

• cargo +1.95.0 test -p arrow-csv --lib --quiet: 96/96 passed.
• cargo +1.95.0 clippy -p arrow-csv --all-targets --quiet: passed with pre-existing warnings outside this diff.
• git diff --check: passed.
• Independent read-only review: approved.