refactor: rebuild exchange data pipeline with full ISO 10383 MIC list

- Regenerate exchange_data.json with all 2,272 active MICs (operating + segments)
- Remove country and city fields from exchange data (out of scope)
- Remove country/city properties from Exchange type and ExchangeData TypedDict
- Rewrite update script to use stdlib only (csv, urllib) — no pandas/openpyxl/requests
- Update tests to reflect simplified data model
- Source: https://www.iso20022.org/market-identifier-codes

Author: sandole