Refactor ePSF building

[larrybradley]

This large PR is a complete refactor of the ePSF building tools.

Summary of user-facing changes:

• ImagePSF now has a shape property
• EPSFBuilder now automatically excludes stars that repeatedly fail fitting and emits warnings with specific failure reasons
• Removed the norm_radius keyword from EPSFBuilder
• Removed the build_epsf method from EPSFBuilder. Use the callable interface (builder(stars)) instead
• Removed the deprecated FittableImageModel and EPSFModel classes. Use ImagePSF instead.
• EPSFBuilder now returns an EPSFBuildResult dataclass containing the ePSF, fitted stars, iteration count, convergence status, and excluded star diagnostics. Tuple unpacking is still supported for backward compatibility.
• LinkedEPSFStar no longer inherits from EPSFStars