Fix Unity error message interpolation

[sylvesterkaczmarek]

Summary

Fix two Unity error paths that currently expose literal {...} placeholders instead of the values that are meant to help diagnose the failure.

The changes:

• interpolate the measured fromto vector magnitude and tolerance in MjEngineTool.ParseFromToMjcf
• interpolate Model.njmax in the CNSTRFULL PhysicsRuntimeException

There is no behavioural change outside the diagnostic text.

Validation

• Based directly on current upstream main (80d1b0d81148e4f85b0260bc0d5f0ca92ca52113).
• Final branch contains one commit (5b4c0eb6dcd05da67247f955c654adec70c0bdbc) and changes exactly two C# lines.
• git diff --check passes.
• Unity EditMode tests were not executed locally, so no Unity test pass is claimed.