refactor: split playback proxy logic into separate handler files

internal/playback/handler/proxy_headers.go

@@ -0,0 +1,17 @@
+package handler
+
+import "net/http"
+
+func copyProxyHeaders(dst http.Header, src http.Header) {
+	// Skip hop-by-hop headers; see RFC 7230 section 6.1.
+	// We intentionally preserve multi-value headers by copying the full slice.
+	for k, v := range src {
+		switch http.CanonicalHeaderKey(k) {
+		case "Connection", "Keep-Alive", "Proxy-Authenticate", "Proxy-Authorization", "Te", "Trailer", "Transfer-Encoding", "Upgrade":
+			continue
+		}
+		copied := make([]string, len(v))
+		copy(copied, v)
+		dst[k] = copied
+	}
+}