GIBS capabilities list .svg legend URLs, but the Anthropic API only
accepts raster image types. GIBS hosts PNG versions at the same path,
so we rewrite .svg → .png and validate content-type on both legend
fetch paths (direct URL and WMS GetLegendGraphic fallback).
Tests: 79 total (32 new), covering LRU cache eviction, BBox
validation, _parse_rgb error handling, colormap ID extraction,
client retry logic, WMS response validation, dimension clamping,
geocode caching, classification colormaps, and scientific notation
interval parsing.
Shutdown: register atexit handler to close httpx connection pool
when the MCP server exits, since FastMCP Middleware has no
on_shutdown hook.
