fix: validate response data inputs

[kholdrex]

Summary

Adds constructor-time validation for response data passed to the Rasch, 2PL, and 3PL models. The models now reject malformed input with clear ArgumentError messages instead of allowing ambiguous failures or invalid fits.

Changes

• Adds a shared response data validator for all three model constructors.
• Requires response data to be a non-empty rectangular Matrix or array-of-arrays.
• Allows only integer 0, integer 1, and nil response values.
• Documents the stricter input contract in the README and changelog.
• Adds shared specs for invalid shapes, invalid values, direct type rejection, and strict binary response handling.

Test plan

• RSpec suite passed.
• RuboCop passed with no offenses.
• Direct model-file require smoke checks passed.
• Gem build/package check passed.