Warn when sysid parameters have no modifier

[sylvesterkaczmarek]

Summary

Make a silent sysid configuration failure visible when a non-frozen Parameter participates in the decision vector but has modifier=None.

This is the failure mode behind #3286: a modifier factory can accidentally return None, leaving the optimized value disconnected from the MjSpec. The optimizer then sees a zero gradient and can terminate immediately without explaining why.

This change:

• checks non-frozen parameters when apply_param_modifiers_spec is used
• emits a warning naming any parameter whose modifier is None
• emits that warning only once per ParameterDict, avoiding log spam during repeated residual evaluations
• keeps frozen parameters silent
• does not change custom build-model workflows unless they explicitly use apply_param_modifiers_spec
• adds regression coverage for the warned and frozen cases

Addresses #3286.

Validation

The final branch is based directly on current main and contains one focused commit changing only model_modifier.py plus regression tests.

The full sysid test environment was not executed locally here, so no local test pass is claimed.