Add unit tests for MQTTPropAdd buffer and packet size limit error paths

[guclumhg]

Description

Adds six unit tests covering previously untested error paths in core_mqtt_prop_serializer.c:

• Buffer-full-at-currentIndex conditions in the addPropUint8/addPropUint16/addPropUint32 helpers (exercised through MQTTPropAdd_PayloadFormat, MQTTPropAdd_TopicAliasMax and MQTTPropAdd_SessionExpiry) and in MQTTPropAdd_SubscriptionId: the buffer is large enough overall, but currentIndex leaves fewer bytes than needed to encode the property.
• The maximum MQTT remaining length (268,435,455) limit checks in MQTTPropAdd_SubscriptionId.
• The 16-bit key/value length validation in MQTTPropAdd_UserProp.

Coverage impact (lcov, branch coverage enabled):

• core_mqtt_prop_serializer.c: lines 98.5% -> 100%, branches 171/182 (94.0%) -> 180/182 (98.9%)
• Total: branches 93.8% -> 94.1%

The two remaining branch directions in addPropUtf8 (the propertyLength > SIZE_MAX - 2 and propertyLength + 2 > 268435456 conditions) require propertyLength > 65535, which cannot be exercised by a test that passes the assert-enabled build due to assert( !CHECK_SIZE_T_OVERFLOWS_16BIT( propertyLength ) ); the public wrappers additionally validate the same bound before calling the helper.

Test Steps

Both CI unit-test configurations pass locally (5/5 test suites):

cmake -S test -B build -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Debug \
  -DBUILD_CLONE_SUBMODULES=ON -DUNITTEST=1 [-DCOV_ANALYSIS=1] \
  -DCMAKE_C_FLAGS='--coverage -Wall -Wextra -Wsign-compare -Werror -DLIBRARY_LOG_LEVEL=LOG_DEBUG'
make -C build all
ctest --test-dir build -E system --output-on-failure
make -C build coverage

Formatting verified with uncrustify 0.69.0 using the config from FreeRTOS/CI-CD-Github-Actions.

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.

Related Issue

N/A

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