test: use constructor seeds in model specs

[kholdrex]

Summary

Model determinism specs now exercise the public constructor seed: option instead of resetting Ruby's global random number generator. The large generated datasets in the model specs also use local random generators so those smoke fixtures remain reproducible without polluting process-wide RNG state.

Changes

• Updated Rasch, 2PL, and 3PL per-model deterministic examples to pass seed: directly to constructors.
• Replaced global random fixture generation with local Random.new instances.
• Seeded the corresponding model construction in large-dataset smoke tests for easier failure reproduction.

Test plan

• Targeted model specs passed for Rasch, 2PL, and 3PL.
• Full RSpec suite passed.
• RuboCop passed with no offenses.
• Gem package build completed successfully.