[Bugfix] use effort_limit from USD asset if not specified in cfg

[JuanaDd]

Description

This PR fixes a bug in actuator initialization where effort limits specified in USD assets were being incorrectly overridden with a very large default value (1.0e9) for explicit actuator models.

Fixes # (issue)

Previously, the ActuatorBase initialization logic would unconditionally fall back to _DEFAULT_MAX_EFFORT_SIM (1.0e9) for explicit actuator models when effort_limit_sim was not explicitly set in the configuration, even when the USD asset contained finite, meaningful effort limit values.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[Mayankm96]

Modifying the code where we set effort_limit might be simpler i think

        # For explicit models, if user does not provide effort limit, we want to default to USD values.
        # For implicit models, we want to enforce the effort limit through the solver.
        if not self.is_implicit_model and self.cfg.effort_limit_sim is None:
            self.effort_limit = self._parse_joint_parameter(self.cfg.effort_limit, effort_limit)
        else:
            self.effort_limit = self._parse_joint_parameter(self.cfg.effort_limit, self.effort_limit_sim)

[JuanaDd]

@Mayankm96 clone is required because effort_limit would be overwritten to effort_limit_sim again here