[QA] Add tests for payload handler

[mayaparov]

Summary

Add deterministic unit tests for PayloadHandler (OpenCDA ↔ Artery payload building/parsing) using fully stubbed protobuf modules and controlled imports, keeping the suite CI-friendly and independent from protoc output at runtime.

This PR contains tests only and does not modify payload_handler.py.

Motivation

PayloadHandler is responsible for assembling outgoing OpenCDA messages and decoding incoming Artery messages. The logic is stateful (it accumulates per-tick data) and relies on serialization (pickle) to transport structured dictionaries through protobuf byte fields. This is easy to break silently during refactors.

Additionally, the module imports protobuf code at import time and mutates sys.path, which makes naive unit tests fragile and dependent on whether generated *_pb2.py files exist locally. The goal of this PR is to lock down contract-level behavior with fast unit tests that run in minimal CI environments.

Changes

New unit tests

opencda/core/common/communication/test/test_payload_handler.py adds contract-level tests for:

• both context-manager APIs (handle_opencda_payload, handle_artery_payload):
  • deterministic initialization of nested dict structures;
  • persistence of mutations across context exits;
  • isolation across entity IDs and ego IDs.
• make_opencda_message():
  • empty state produces a message with zero entities;
  • multi-entity messages validated without ordering assumptions;
  • auxillary is bytes-like and decodes back to the original per-entity dict via pickle.loads (without comparing raw pickle bytes).
• make_artery_payload():
  • supports multiple transmissions (multiple ego IDs) and multiple entities per transmission;
  • accumulates and overwrites per (ego_id, entity_id) as expected for dict-based storage;
  • does not unexpectedly clear existing state when receiving an empty message.
• clear_messages():
  • resets internal stores to avoid data leakage between ticks.

Notes on planned changes

The current implementation uses pickle for serialization. If the project later replaces pickle with a safer serialization format, these unit tests will need to be updated accordingly (the tests intentionally assert decoded content, not raw serialized bytes).

How to run

uv run pytest -q opencda/core/common/communication/test/test_payload_handler.py
uv run pytest -q