Add const validation keyword

[crmne]

Closes #34

Summary

• Add const: support to primitive schema builders
• Preserve explicit false and nil const values
• Cover string, number, integer, boolean, and null schemas with specs

Verification

• bundle exec rake

[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.75%. Comparing base (f70d91f) to head (7ab8ada).

Files with missing lines	Patch %	Lines
lib/ruby_llm/schema/dsl/schema_builders.rb	96.15%	1 Missing ⚠️

Additional details and impacted files

@@                            Coverage Diff                            @@
##           issue-33-exclusive-numeric-boundaries      #46      +/-   ##
=========================================================================
- Coverage                                  94.79%   94.75%   -0.05%     
=========================================================================
  Files                                         12       12              
  Lines                                        365      381      +16     
  Branches                                      73       75       +2     
=========================================================================
+ Hits                                         346      361      +15     
- Misses                                        19       20       +1     

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