chore: add additionalProperties: false to datasource schema

[firstdata-dev]

Problem

The datasource JSON schema does not set additionalProperties: false, which means files with non-standard field names (e.g. country_code instead of country, coverage instead of geographic_scope) pass make validate silently.

This caused 15 files across PRs #51-#59 to use incorrect field names, which was only caught during manual code review (credit: 明鉴).

Proposed Fix

1. Add "additionalProperties": false to firstdata/schemas/datasource-schema.json
2. Ensure CI validation catches any non-schema fields going forward

Impact

• Prevents silent schema violations
• Catches field name typos at CI time instead of review time