fix(elixir): validate string lengths

[schani]

Summary

• enforce JSON Schema minLength and maxLength in generated Elixir decoders
• use String.length/1 so validation counts Unicode code points rather than bytes
• enable the existing minmaxlength schema fixture for Elixir

Validation

• npm run build
• npm run lint
• FIXTURE=schema-elixir npm run test:fixtures -- test/inputs/schema/minmaxlength.schema test/inputs/schema/optional-constraints.schema

Production change: 12 additions, 3 deletions.

[github-actions]

Generated-output differences

66 files differ — 66 modified, 0 new, 0 deleted
2944 changed lines — +2573 / −371

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