Add Modbus function 16 word order regression coverage

[ilyar]

Summary

Adds regression coverage for Modbus RPC writes using function code 16 with multi-register values.

The tests verify that wordOrder is preserved when values are encoded for device RPC writes:

• 32int with BIG word order writes [0x1234, 0x5678]
• 32int with LITTLE word order writes [0x5678, 0x1234]
• 32float with BIG word order writes [0x4145, 0x70A4]
• 32float with LITTLE word order writes [0x70A4, 0x4145]

The routing-level test also uses the issue-shaped setValue configuration with functionCode: 16, objectsCount: 2, and address: 4.

This covers the behavior reported in #1275 without changing runtime logic. Current master already produces the expected register order for this path, so the PR documents and protects the behavior with focused unit tests.

Validation

• .venv/bin/python -m pytest tests/unit/connectors/modbus/test_device_server_side_rpc.py - 11 passed
• .venv/bin/python -m pytest tests/unit/connectors/modbus - 34 passed

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