refactor(server): replace rawHTTPResponseWriter with stdlib http.Server

- Rewrite server to use net/http.Server with ServeHTTP handler instead
  of raw TCP listener with hand-written HTTP responses. Control plane
  errors now get proper Content-Type, Content-Length, and chunked
  encoding via http.Error(). Tunnel traffic hijacks the connection and
  re-serializes the request for forwarding.
- Simplify reconstructedConn to accept variative io.Readers
- Delete raw_http_response_writer.go (no longer needed)
- Fix TCP forwarder: bare host:port (e.g. "127.0.0.1:5432") now works
  correctly instead of failing on url.Parse. Only HTTP/HTTPS schemes
  go through URL parsing; everything else is treated as raw TCP.
- Add 8 new e2e tests: HTTP response quality, 1MB response body, 512KB
  request body, TCP echo, TCP large payload, concurrent single-tunnel,
  concurrent multi-tunnel (16 tests total, all passing)

server/reconstructed_conn.go

@@ -1,7 +1,6 @@
 package server
 
 import (
-	"bytes"
 	"io"
 	"net"
 )
@@ -14,17 +13,17 @@ type reconstructedConn struct {
 	reader io.Reader
 }
 
-// Read reads from the reconstructed reader (captured data + original conn).
+// Read reads from the reconstructed reader (prepended data + original conn).
 func (rc *reconstructedConn) Read(p []byte) (n int, err error) {
 	return rc.reader.Read(p)
 }
 
-// newReconstructedConn creates a reconstructed connection that replays captured data
-// before reading from the original connection.
-func newReconstructedConn(conn net.Conn, capturedData *bytes.Buffer) net.Conn {
-	allReader := io.MultiReader(capturedData, conn)
+// newReconstructedConn creates a reconstructed connection that replays the provided
+// readers in order before reading from the underlying connection.
+func newReconstructedConn(conn net.Conn, readers ...io.Reader) net.Conn {
+	allReaders := append(readers, conn)
 	return &reconstructedConn{
 		Conn:   conn,
-		reader: allReader,
+		reader: io.MultiReader(allReaders...),
 	}
 }