fix: NTP packet correctness and UDP socket lifecycle

[aharshac]

Summary

• Zero-initialize NTP packet buffer:

  • The previous code cast a 4-byte magic constant to a 48-byte write, sending uninitialized stack memory as NTP timestamp fields.
  • Servers interpret those bytes as offsets and return wrong time.
  • Replaced with a byte packetBuffer[NTP_PACKET_SIZE] = {0} and explicit header-byte assignments.
  • Response is now read into the same buffer via indexed access.
  • Confirmed fix for ESP8266 hardware (reported in PR Make NTP request more robust. #7).

• Close UDP socket on every exit path:

  • The socket opened by mUdp->begin() was never closed, causing begin() to fail when a second client was created on the same UDP object.
  • mUdp->stop() is now called before every return and after the successful response read.

• Remove static from socket initializer:

  • static int udpInited was evaluated exactly once per program lifetime.
  • After the socket is closed by the previous fix, subsequent calls need to re-run begin().
  • Removing static makes getServerTime() fully self-contained: it opens a socket, uses it, and closes it on every call.
  • Local port changed from the hardcoded 123 to NTP_REQUEST_PORT (1123).

Adds test_basic_sync() and test_client_reuse() to examples/TestNodeMCU/TestNodeMCU.ino.

Closes PRs #6 & #7