fix: preserve PERPLEXITY_BASE_URL query parameters when joining paths - #155

makeApiRequest builds the request URL as ${PERPLEXITY_BASE_URL}/${endpoint}, so a query string in PERPLEXITY_BASE_URL swallows the endpoint path: https://proxy.example.com/api/?key=x becomes https://proxy.example.com/api/?key=x/v1/agent.

This parses the base URL and appends the endpoint to its pathname, which keeps the base query intact and avoids a double slash when the base URL ends in /. Base URLs without a query or trailing slash produce the same URL as before.

Same issue as perplexityai/perplexity-py#139.

Validation: new test fails before the change and passes after. tsc --noEmit passes, and vitest run passes apart from proxyAwareFetch > should NOT use native fetch when proxy is configured, which also times out on main in my environment.