Fix MQTT client: reset use_tls flag in connection_end (TLS to plain reconnect)

[EdouardMALOT]

Summary

The MQTT client cannot make a non-secure connection after a secure one on the same client instance.

_nxd_mqtt_client_secure_connect() sets nxd_mqtt_client_use_tls = 1 before delegating to _nxd_mqtt_client_connect(), but the flag is never cleared anywhere — _nxd_mqtt_client_connection_end() ends and deletes the TLS session, yet leaves the flag set. A subsequent nxd_mqtt_client_connect() (plain) on the same client inherits the stale flag, calls nx_secure_tls_session_start() on the TLS session that was just deleted, and fails with NXD_MQTT_CONNECT_FAILURE (0x10005).

Typical scenario: a device provisioned over TLS that must fall back to (or be reconfigured for) a plain connection without recreating the NXD_MQTT_CLIENT instance.

Changes

• addons/mqtt/nxd_mqtt_client.c — in _nxd_mqtt_client_connection_end(), clear nxd_mqtt_client_use_tls after the TCP socket is disconnected and unbound.

The placement matters: clearing the flag before the socket teardown opens a window where a late TLS record (e.g. the peer's close_notify) is read through the plain TCP path and parsed as MQTT bytes; its fixed header bytes decode as a truncated MQTT message that gets parked forever in nxd_mqtt_client_processing_packet, leaking a packet (caught by netx_mqtt_packet_leak_test in the require_secure_build configuration). With the flag cleared only after nx_tcp_client_socket_unbind(), any late TLS data keeps failing through the TLS receive path exactly as it does today.

_nxd_mqtt_client_connection_end() is only reached on terminal paths (connect failure, disconnect processing, timeout, explicit disconnect); every reconnection goes back through nxd_mqtt_client_connect() or nxd_mqtt_client_secure_connect(), and the latter re-runs the TLS setup callback and sets the flag again — so secure reconnection is unaffected.

Behavioral note: an application that (incorrectly) called the plain nxd_mqtt_client_connect() to re-establish a previously secure session used to get NXD_MQTT_CONNECT_FAILURE; it now gets exactly what it requested — a plaintext connection — matching the documented semantics of the two connect APIs.

Test plan

• secure connect → disconnect → plain connect on the same client: previously failed with 0x10005, now succeeds
• secure connect → disconnect → secure connect: unchanged (TLS setup callback re-runs)
• plain-only and secure-only usage: unchanged
• MQTT regression suite (including netx_mqtt_packet_leak_test in all build configurations)