fix: validate encoded telemetry message size

[Carter Tinney (cartertinney)]

Summary

• replace sys.getsizeof() telemetry validation with deterministic IoT Hub message accounting: encoded body bytes, outgoing system-property values, and custom-property names and values
• mirror Paho's payload conversion (str as UTF-8, int/float as ASCII, None as empty, and bytes/bytearray as-is) while excluding MQTT topic and packet overhead
• share system/custom property normalization with MQTT topic encoding and include the module output system property before validation
• replace Python-layout-dependent magic numbers and Python 3.12+ skips with exact below/equal/above 262144-byte tests for Unicode, bytes, custom properties, and system properties across sync/async device/module sends

Behavior and compatibility

IoT Hub defines message size independently of the transport as body bytes plus system-property values plus application-property names and values. The old implementation measured CPython object allocation instead, so identical messages could validate differently across Python versions.

This change intentionally affects messages near the 256-KB boundary:

• exactly 262144 logical message bytes remain accepted; larger messages are rejected
• UTF-8 strings and raw byte payloads are measured as actually published by Paho
• custom-property keys now contribute to size, while Python object headers no longer do
• outgoing system-property values, including an IoT Edge module output name, contribute to size
• percent-encoding, the base MQTT topic, and MQTT framing do not contribute because they are protocol overhead rather than IoT Hub message content
• payload types unsupported by Paho now fail during size validation with TypeError instead of later in the publish call; the accepted payload type set is unchanged

The MQTT topic encoding itself is unchanged; sizing and encoding now consume the same normalized property values to prevent drift.

Validation

• focused message/topic/sync/async send tests: 178 passed on Python 3.10, 3.11, 3.12, 3.13, and 3.14
• full unit suite on Python 3.10: 5463 passed
• full unit suite on Python 3.13: 5463 passed
• full unit suite on Python 3.14: 5463 passed
• full Python 3.11 runs reached 5462/5461 passes with only pre-existing short timing-window failures in test_alarm.py and test_sync_handler_manager.py; every failure passed immediately in isolation
• full Python 3.12 run reached 5462 passes with only a pre-existing test_alarm.py timing-window failure; it passed immediately in isolation
• changed files pass flake8 and git diff --check

[Copilot]

Pull request overview

Replaces Python object-size telemetry validation with deterministic IoT Hub byte accounting.

Changes:

• Measures encoded payload and normalized property sizes.
• Includes module output names before validation.
• Adds exact 256-KB boundary coverage for sync and async clients.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
azure-iot-device/azure/iot/device/iothub/models/message.py	Implements deterministic message sizing.
azure-iot-device/azure/iot/device/iothub/pipeline/mqtt_topic_iothub.py	Shares property normalization with sizing.
azure-iot-device/azure/iot/device/iothub/sync_clients.py	Includes output name in sync validation.
azure-iot-device/azure/iot/device/iothub/aio/async_clients.py	Includes output name in async validation.
tests/unit/iothub/models/test_message.py	Tests encoding and size boundaries.
tests/unit/iothub/test_sync_clients.py	Tests synchronous boundary validation.
tests/unit/iothub/aio/test_async_clients.py	Tests asynchronous boundary validation.

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[Ewerton Scaboro da Silva (ewertons)]

Excellent!