fix: update WiFi example for new Zephyr APIs, fix memory leak, missing headers and smaller things

[paulober]

Overview

This PR updates the WiFi example so it builds and runs correctly on current Zephyr versions.
It fixes broken POSIX usage, missing headers, DNS memory leaks, and API changes introduced upstream.

• POSIX API cleanup

  • Correct use of Zephyr’s POSIX layer (getaddrinfo, socket, connect, etc.)
  • Added required headers and cleaned include order.
  • Removed unused headers.

• prj.conf updates

  • Added CONFIG_POSIX_API=y to enable POSIX declarations.

• Memory leak fix

  • Added freeaddrinfo() in all exit paths.
  • Ensured sockets are properly closed on errors.

• Networking updates

  • Updated ICMP/ping code to current APIs.
  • WiFi example now sets correct PSK, band, and parameters.
  • Pin used wifi band and dropped unused enterprise settings

• General cleanup

  • Some minor issues.
  • Improved code formatting.
  • Removed unused or outdated code.

Result

The full example now builds cleanly on Zephyr ≥ 4.3 and successfully performs WiFi connection, DHCP, DNS, HTTP GET/POST, JSON parsing, and ICMP ping on Raspberry Pi Pico 2 W.

[jancumps]

This pull request mixes code changes with reformatting of unchanged code. Makes it harder to understand what's fixing problems.