eds: make Size method return error

[VolodymyrBg]

The Size method in the eds.Accessor interface now returns an error to handle cases where:

• The header is not initialized in ODS
• The extended data square is not initialized in Rsmt2D
• The accessor is closed in closeOnce

This change makes the interface more consistent with other methods that return errors
and allows proper error handling in cases where size information cannot be retrieved.

Updates:

• eds.Accessor interface to include error in Size method signature
• All implementations (ODS, ODSQ4, Rsmt2D, NoopFile, validation, proofsCache, closeOnce)
• Tests and mocks to handle the error return

Resolves TODO in store/file/ods_q4.go about Size method returning error.

[walldiss]

Thank you for contribution