Migrate httputil.ReverseProxy from Director to Rewrite

[zackverham]

Summary

This PR fixes a go deprecation that was introduced by go 1.26. Interestingly, it looks like there was never any test coverage for the proxy package, so this PR also adds test coverage we were missing.

---

• Migrates from the deprecated Director field to the newer Rewrite function in httputil.ReverseProxy
• Director was deprecated in Go 1.26
• The Rewrite function provides cleaner separation between inbound and outbound requests via the ProxyRequest type
• Adds comprehensive test coverage for the proxy package (previously had none)

Changes

• Replace Director field with Rewrite function
• Remove baseProxy intermediate proxy (no longer needed)
• Store targetURL as *url.URL instead of string
• Use pr.SetURL() for URL rewriting instead of calling base Director
• Inline stripSourcePrefix logic into rewrite() and proxyURL()

Key Differences

Aspect	Director (old)	Rewrite (new)
Signature	func(req *http.Request)	func(pr *httputil.ProxyRequest)
Request access	Single req (clone)	pr.In (original), pr.Out (to modify)
URL rewriting	Call base Director	Use pr.SetURL(targetURL)

Test Coverage Added

Added 17 tests covering all proxy functionality:

Test	Description
TestBasicProxy	Verifies requests are forwarded and responses returned
TestPathStripping	Verifies sourcePath is removed from request paths
TestPathStrippingToRoot	Verifies empty path becomes "/"
TestQueryStringPreserved	Verifies query parameters pass through
TestCookiesStripped	Verifies cookies are not forwarded (security)
TestHostHeaderSet	Verifies Host header is set to target
TestRefererRewritten	Verifies Referer header is rewritten
TestRefererEmpty	Handles missing Referer gracefully
TestLocationHeaderRewritten	Verifies redirect locations are rewritten
TestLocationHeaderRelativeUnchanged	Relative redirects pass through unchanged
TestErrorHandler	Verifies 502 on backend errors
TestPostRequestBody	Verifies POST bodies are forwarded
TestProxyURL	Tests the proxyURL() helper
TestProxyURLWithQueryString	Tests query string handling in helper
TestProxyURLToRoot	Tests root path mapping
TestProxyURLInvalidURL	Tests error handling for invalid URLs
TestOtherHeadersPreserved	Verifies non-cookie headers pass through

Test Plan

• Code compiles successfully
• Full test suite passes
• New proxy tests pass (17/17)
• Manual testing of proxy functionality

Closes #3572

🤖 Generated with Claude Code