[log] Add debug logging to proxy upstream forwarding (#14309)

Adds 5 debug log calls to `internal/proxy/proxy.go`, reusing the
existing `logProxy` logger:

- `upstreamHost`: scheme-less parse retry and raw host fallback
- `forwardToGitHub`: GraphQL URL rewrite (GHES or not), request creation
failure, artifact zip no-redirect path

The log arguments have no side effects. They don't log paths, URLs or
credentials, so enclave and delegation modes stay safe.

Validation: `go vet` and `go test ./internal/proxy` pass, and `go build`
succeeds.

> Generated by [Go Logger
Enhancement](https://github.com/github/gh-aw-mcpg/actions/runs/37077512907)
· copilot · auto · 62.4 AIC · ⊞ 12.8K ·
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Author: lpcox

internal/proxy/proxy.go

@@ -457,12 +457,14 @@ func (s *Server) upstreamHost() string {
 	}
 
 	// Handle scheme-less config values like "api.github.com" or "api.github.com/api/v3".
+	logProxy.Print("upstreamHost: githubAPIURL has no host, retrying parse as scheme-less value")
 	u, err = url.Parse("https://" + strings.TrimLeft(s.githubAPIURL, "/"))
 	if err == nil && u.Host != "" {
 		return u.Hostname()
 	}
 
 	host, _, _ := strings.Cut(strings.TrimLeft(s.githubAPIURL, "/"), "/")
+	logProxy.Print("upstreamHost: falling back to raw host segment")
 	return host
 }
 
@@ -483,6 +485,7 @@ func (s *Server) forwardToGitHub(ctx context.Context, method, path string, body
 			graphqlURL = s.githubAPIURL + "/graphql"
 		}
 		url = graphqlURL
+		logProxy.Printf("forwardToGitHub: rewrote GraphQL upstream URL: ghes=%v", strings.HasSuffix(s.githubAPIURL, "/api/v3"))
 		if hasQuery {
 			url += "?" + query
 		}
@@ -497,6 +500,7 @@ func (s *Server) forwardToGitHub(ctx context.Context, method, path string, body
 
 	req, err := http.NewRequestWithContext(ctx, method, url, body)
 	if err != nil {
+		logProxy.Printf("forwardToGitHub: failed to create upstream request: method=%s, err=%s", method, sanitize.RedactErrorForLog(err))
 		return nil, fmt.Errorf("failed to create upstream request: %w", err)
 	}
 
@@ -514,6 +518,7 @@ func (s *Server) forwardToGitHub(ctx context.Context, method, path string, body
 		req.Header.Set("Content-Type", contentType)
 	}
 	if method == http.MethodGet && artifactZipDownloadPathPattern.MatchString(pathOnly) {
+		logProxy.Print("forwardToGitHub: artifact zip download, not following redirects")
 		return doWithoutFollowingRedirects(s.httpClient, req)
 	}
 	return s.httpClient.Do(req)