Reject unsupported salt costs - #302

Summary

Raise BCrypt::Errors::InvalidCost when BCrypt::Engine.generate_salt receives a cost above MAX_COST.

On MRI, unsupported costs currently reach the native salt generator and return nil; JRuby raises from its Java implementation. Validating the documented upper bound in the shared Ruby layer gives both engines a deliberate bcrypt error and prevents a nil salt from escaping the public method. This is the same boundary originally reported in #27.

Reproduction

On bcrypt 3.1.22, BCrypt::Engine.generate_salt(32) returns nil on MRI. An external model checks costs 32, 99, and 2**30; all now raise BCrypt::Errors::InvalidCost, while the existing sub-minimum clamp and Password.create upper-bound behavior remain unchanged.

Verification

• bundle exec rake compile spec: 39 examples, 0 failures on Ruby 4.0.6
• focused upper-bound model: pass

Compatibility

Valid costs are unchanged. The only behavior change is replacing an unusable nil result or engine-specific native exception for costs above 31 with the gem's documented invalid-cost error.