Add optional name parameter to PyMCStateSpace for multiple models

[opherdonchin]

Summary

As discussed in [#602](#602), this PR adds an optional name parameter to PyMCStateSpace. When provided, it is used to prefix all variables and registered data created by that state-space model. This makes it possible to use multiple state-space models inside one PyMC model without name collisions.

What this changes

• Add name: str | None = None to PyMCStateSpace.__init__.

• Add _prefix_name(self, var_name: str) -> str, which:

  • returns var_name unchanged if self.name is None
  • otherwise returns "{name}_{var_name}".

• Apply _prefix_name when:

  • creating pm.Deterministic variables,
  • registering Kalman filter and likelihood outputs,
  • constructing names in SequenceMvNormal,
  • naming H_jittered, x0_slice, P0_slice, forecast_*, irf, initial_shock,
  • building names for posterior predictive variables.

• Update data helpers (add_data_to_active_model, register_data_with_pymc) so that state-space models can optionally pass prefixed data names.

Backwards compatibility

• If name is not provided, everything behaves exactly as before.
• Existing code that uses a single PyMCStateSpace should work without any changes.

Example

ssm1 = PyMCStateSpace(..., name="vision")
ssm2 = PyMCStateSpace(..., name="motor")

This results in variables such as "vision_obs", "motor_obs", "vision_data", etc., allowing both models to coexist cleanly in the same PyMC model.

Testing

• Added tests/statespace/test_register_variable.py to check the basic naming and variable-registration behavior.
• Attempted to run pytest tests/statespace in a clean pip install -e ".[dev]" environment.
• Test collection currently stops with FutureWarning errors from pytensor.graph.basic.* imports (these have moved to pytensor.graph.traversal.*). This appears unrelated to this PR and originates from existing imports in the codebase and test suite.