Add Xpress 9.8+ API support with backward compatibility

[brunoficoxpress]

Summary

This PR updates the linopy Xpress solver integration to support the new Python API introduced in Xpress 9.8+, while maintaining full backward compatibility with Xpress 9.6+ through fallback mechanisms.

Changes

1. API Method Migration (linopy/solvers.py)

All method calls updated with try/except blocks for backward compatibility:

• m.read() → m.readProb()
• m.setlogfile() → m.setLogFile()
• m.readbasis() → m.readBasis()
• m.writebasis() → m.writeBasis()
• m.writebinsol() → m.writeBinSol()
• m.postsolve() → m.postSolve()
• m.getnamelist() → m.getNameList()
• m.getDual() → m.getDuals()

2. Status Handling Improvements (linopy/solvers.py)

• Replaced numeric status codes with xpress.SolStatus enum constants
• Updated solver status checks to use xpress.enums.SolveStatus.STOPPED
• Improved code readability and maintainability

3. Exception Handling (linopy/solvers.py)

• Replaced broad Exception catching with specific xpress.SolverError and xpress.ModelError
• Prevents masking of unrelated errors while catching only solver-specific issues

4. IIS (Irreducible Infeasible Subset) Detection (linopy/model.py)

• solver_model.iisall() → solver_model.IISAll() with fallback
• solver_model.getiisdata() → solver_model.getIISData() with fallback
• Fixed mypy type checking errors by declaring variables before try/except blocks

5. Unicode Handling (linopy/constraints.py)

• Added Windows console encoding fix for Unicode characters (≤, ≥) in constraint display

6. Documentation Updates

• Updated Xpress documentation URLs in doc/index.rst and doc/prerequisites.rst

Testing

Test Environment

• Xpress 9.8.0: Primary development version
• Xpress 9.6.0: Backward compatibility validation

Test Results

• ✅ All 414 Xpress-specific tests PASSED
• ✅ All infeasibility detection tests PASSED (IIS functionality verified)
• ✅ Pre-commit hooks (Black, Ruff): All passed
• ✅ Backward compatibility confirmed through fallback mechanisms