[Core] Enable analysis stage without solver

[rubenzorrilla]

📝 Description

Description

This PR enables running AnalysisStage without requiring a solver by introducing a Null Object pattern as more clean alternative to #14150.

Previously, _CreateSolver() was expected to always return a valid solver and raised an exception otherwise. This prevented the implementation of analysis stages that do not require a solver. A PythonNullSolver is added and used as the default solver in the base AnalysisStage. This allows solver calls to be executed unconditionally while safely performing no-ops when no solver is required.

With this change:

• Solver becomes optional by design.
• The execution pipeline remains unchanged (no more changes in AnalysisStage).
• No additional branching (i.e., if solver is not None) is introduced

In short, only change is that solver is now always present (real or "null").

Implementation details

• Added python_null_solver.py containing PythonNullSolver and its singleton instance PYTHON_NULL_SOLVER to enforce statelessness and avoid allocations.
• Modified AnalysisStage._CreateSolver() to return PYTHON_NULL_SOLVER by default
• Kept lazy initialization in _GetSolver()
• Added _HasSolver() utility method to distinguish between real and null solver
• PythonNullSolver dynamically handles any method via __getattr__, returning a no-op callable. This enables dynamic handling of Python solver interface (just in case this changes in the future).
• Keeps full backward compatibility for existing solver-based stages overriding the _CreateSolver() method.

Tests have been added accordingly.

[loumalouomega]

CI is failing

[rubenzorrilla]

CI is failing

It is because I also took the chance to remove the 5 years old deprecated handling of output processes outside the corresponding output_processes block. Some old tests were relying on it (something I didn't expect to happen...). I'll do this in a separate PR.

[loumalouomega]

Once Ci passes ping me and I will approve

[roigcarlo]

LGTM 👍 . But not to the CI