fix: preserve base URL query parameters when joining paths

[qirh]

A custom baseURL that contains a query string corrupts the endpoint path. buildURL concatenates the request path onto the full base URL string, so https://proxy.test/api?key=x plus /search becomes https://proxy.test/api?key=x/search: the request goes to /api with key=x/search.

This splits the base query off before joining paths, then merges it into the request query with the lowest precedence (base, then path, then defaultQuery, then per-request query). Absolute request paths are unchanged.

Same issue as perplexityai/perplexity-py#139.

Validation: new baseUrl > query parameters test fails before the change and passes after. bazel test //... passes (8/8), plus oxlint, oxfmt and Gazelle diff checks.