Stop hard-coding the flepimop2-op_system module version

[jc-macdonald]

Problem

flepimop2-op_system/pyproject.toml is at 0.4.0, but flepimop2.system.op_system.__version__ is still hard-coded to 0.2.0 and exported publicly. This stale value survived the work discussed in #141/#142 and will drift again if it is updated by hand.

Proposed fix

• derive the provider version from installed distribution metadata, or remove the duplicate public constant if it is not part of the intended API
• define source-tree/fallback behavior explicitly
• add a regression test comparing the module value (if retained) with importlib.metadata.version("flepimop2-op-system")
• document any deprecation if the duplicate constant is removed

Keep this independent of the Array callable-contract work in #229/#230.