Fix error message formatting in configuration validation

[emmanuel-ferdman]

Pull Request Description:

This small PR corrects validation error messaging in the QFractionLinearAnnealingConfig class. The PR fixes two issues:

• Missing f-string formatting: Added proper string interpolation to display actual end_step and start_step values.
• Typo in error message variable: Corrected start_stap to start_step to match the actual parameter name, preventing a runtime error.

Checklist before requesting a review:

• I set the appropriate labels on the pull request.
• I have added/updated the release note draft (if necessary).
• I have updated the documentation to reflect my changes (if necessary).
• All function and files are well documented.
• All function and classes have type hints.
• There is a licenses in all file.
• The function and variable names are informative.
• I have checked for code duplications.
• I have added new unittest (if necessary).