app: Move from nrf_ to zsock_ socket operations

[MarkusLassila]

Replace nrf_ socket operations with offloaded zsock_ socket operations.

Note: We changed from zsock_socket to nrf_socket in PR #47 due to SO_POLLCB support being only for nrf_sockets.. Now that we have SO_POLLCB, we can move back to offloaded sockets.

Jira: SM-319

[Copilot]

Pull request overview

This PR migrates the Serial Modem application and its AT-socket test suite from nrf_* socket APIs to offloaded zsock_* socket operations, leveraging SO_POLLCB support to return to offloaded sockets.

Changes:

• Replace nrf_* socket calls with zsock_* equivalents across the AT socket layer and HTTP client paths.
• Update unit tests to mock zsock_* APIs and enable POSIX socket namespace compatibility for the test build.
• Remove direct dependency on nrf_socket.h from shared defines and adjust buffer sizing.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
app/src/sm_at_socket.c	Switch core AT socket operations, sockopts, polling and callbacks to zsock_*.
app/src/sm_at_httpc.c	Use zsock_send/recv and ZSOCK_POLL* events for HTTP client socket I/O.
app/src/sm_at_httpc.h	Update poll-event documentation to reference ZSOCK_POLL*.
app/src/sm_ppp.c	Use zsock_inet_pton for PPP DNS fallback address parsing.
app/src/sm_defines.h	Drop nrf_socket.h include and hardcode SM_MAX_MESSAGE_SIZE.
app/src/sm_at_icmp.c	Remove nrf_socket.h include (keep Zephyr socket usage).
app/tests/at_socket/CMakeLists.txt	Enable CONFIG_NET_NAMESPACE_COMPAT_MODE for the test target.
app/tests/at_socket/src/test_at_socket.c	Update mocks/expectations from nrf_* to zsock_* across test cases.

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