Fix urljoin bug with base URL path prefixes

[adamtheturtle]

Summary

Replace urljoin() with string concatenation to preserve path prefixes in base URLs. This fixes an issue where MockVWS(base_vws_url="http://localhost/prefix") would incorrectly register handlers at http://localhost/targets instead of http://localhost/prefix/targets.

Add comprehensive tests for path prefix handling in both requests_mock and respx implementations. The fix ensures path prefixes are properly preserved when constructing URL patterns for the mock interceptors.

Test plan

• ✅ New tests verify path prefix handling for VWS and VWQ endpoints
• ✅ All 10 tests in TestCustomBaseURLs pass
• ✅ Existing tests without path prefixes continue to work

🤖 Generated with Claude Code

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Note

Low Risk
Small, localized change to URL-pattern construction plus additional tests; main risk is regressions in route matching for edge-case base URLs.

Overview
Fixes mock route registration when base_vws_url/base_vwq_url include a path prefix by replacing urljoin() with explicit base_url.rstrip("/") + route.path_pattern concatenation in both the requests (responses) and httpx (respx) decorators.

Adds coverage to ensure prefixed base URLs only intercept requests under the prefix (and not the root), and updates the httpx adapter to normalize request header casing; also adds an httpx test asserting the VuMark instances endpoint returns a bytes response successfully.

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