Documentation fixes and additions

[AniruddhaKanhere]

• core_mqtt.h: fix MQTT_InitRetransmits example (remove phantom publishClearAllCallback so example matches the 3-callback signature); fix MQTT_InitStatefulQoS docstring (@p pBuffer -> @p pAckPropsBuf); complete the truncated MQTTStatusNotConnected return-value sentence in MQTT_CheckConnectStatus.

• Replace C99 compound literals (&(uint8_t){ MQTT_PACKET_TYPE_* }) in all doc examples with the C90-compatible MQTT_PROP_VALIDATE_* macros across core_mqtt.h (5), core_mqtt_serializer.h (1), MigrationGuide.md (26), and MQTTv5Guide.md (9). The one PUBREL case, which has no matching MQTT_PROP_VALIDATE_* macro, uses MQTT_PROP_NO_VALIDATE.

• README.md: add Thread Safety section documenting the MQTT_PRE_STATE_UPDATE_HOOK / MQTT_POST_STATE_UPDATE_HOOK extension points, with a non-recursive FreeRTOS mutex example (verified: the library does not re-enter a hook-protected region).

• core_mqtt_serializer.h: add an explanatory comment block above the MQTT_PACKET_TYPE_VAL file-scope statics explaining why they exist and why the MISRA C:2012 Rule 2.2 / 2.8 deviations are accepted (macros cannot be addressed, MQTT_PROP_VALIDATE needs a typed const uint8_t *).

• Add source/include/core_mqtt_config_template.h: a commented-out starter template users can copy as core_mqtt_config.h, covering every tunable documented in core_mqtt_config_defaults.h plus the thread-safety hooks.

Description

Test Steps

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

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