[Feature]: Add World Bank Global Economic Indicators Tool (Python)

### Description Create a Python tool in `tools/world_bank_indicators/` that queries the World Bank Open Data API to fetch macroeconomic indicators (GDP growth `NY.GDP.MKTP.KD.ZG`, inflation `FP.CPI.TOTL.ZG`, population `SP.POP.TOTL`) across countries. ### Acceptance Criteria - [ ] Directory `tools/world_bank_indicators/` with `tool.py`, `metadata.json`, `requirements.txt`, and `README.md`. - [ ] Parameters: `country` (ISO2 code, e.g. `US`, `IN`, `GB`), `indicator` (string), `years` (int, default 5). - [ ] Returns chronological observations with year and numerical value. - [ ] Passes `python3 scripts/validate_tools.py`.