[deps] Upgrade PyModbus and migrate the application API #16

Background

The project currently pins PyModbus 3.6.9 in pyproject.toml and requirements.txt. The simulator uses PyModbus APIs that changed in later releases, including datastore contexts and framer classes, so this requires a compatibility migration rather than a version-only bump.

Scope

• Identify the latest stable PyModbus release compatible with the project's supported Python range (>=3.10,<4.0).
• Upgrade the PyModbus constraint and regenerate the lock/exported requirements without changing unrelated dependencies.
• Migrate the simulator to the selected release's supported APIs.
• Preserve CLI behavior, datastore contents/address ranges, and TCP, UDP, TLS, serial, and framer selection behavior.
• Add or update regression coverage where the migration changes behavior, especially for supported transports and clean server shutdown.

Acceptance criteria

• The selected PyModbus version and its Python compatibility are documented in the change.
• pyproject.toml, poetry.lock, and requirements.txt agree on the dependency version.
• The existing tests and make check pass.
• The tox matrix passes on Python 3.10, 3.11, 3.12, 3.13, and 3.14.
• No unrelated dependency upgrades are included.