Validate row field counts before serialization

[nickvanw]

Summary

• require row value counts to match the result field list before building column maps
• return explicit serialization errors for extra or missing values instead of panicking or emitting partial records
• add direct coverage for both mismatch directions

Evidence

The old convertRowToMap check used idx > len(columns), so a row with exactly one extra value would index columns[len(columns)] and panic. Rows with too few values silently omitted columns because serialization iterated over the partial map.

Validation

• GOCACHE=/tmp/fivetran-bugbash.qcjUz1/gocache go test ./cmd/internal/server/handlers -run 'TestConvertRowToMapRequiresMatchingColumnCount'
• GOCACHE=/tmp/fivetran-bugbash.qcjUz1/gocache go test ./cmd/internal/server/handlers
• git diff --check