Add Xquik API example and fix cache refresh duplication

[kriptoburak]

Summary

• add a ready-to-use API_SOURCES example for Xquik's public OpenAPI 3.1 document
• replace an existing source's cache entry during refresh instead of appending duplicates
• add a regression test that refreshes the same OpenAPI source twice

Why this helps

CacheManager.refresh() previously appended another entry for every configured source on each refresh. Long-running MCP sessions could therefore return duplicate documentation and keep growing the in-memory cache. The cache now retains one current entry per source name.

The Xquik example points to the public https://xquik.com/openapi.json document, which currently exposes OpenAPI 3.1.0 with 99 paths.

Validation

• pnpm install --frozen-lockfile
• pnpm run build
• pnpm test (13 tests passed)
• live Xquik OpenAPI parse check (OpenAPI 3.1.0, 99 paths)