fix!: Return disabled config if no defaultValue is provided

[jsonbailey]

feat!: Use kwargs for completion_config and config methods
feat: Drop support for Ruby 3.0 which is EOL.

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Note

Medium Risk
This is a breaking public API change (positional args to kwargs) and alters default fallback behavior to disabled, which could change downstream runtime behavior if callers relied on nil/implicit defaults.

Overview
Breaking API + runtime behavior update for AI config evaluation. Client#completion_config (and deprecated #config) now require keyword arguments (key:, context:, default:, variables:) and will implicitly fall back to a disabled config when no default is provided (via new AIConfig.disabled).

Drops Ruby 3.0 support by bumping the gem’s required_ruby_version to >= 3.1.0, updating RuboCop target, CI/publish workflows, and example Gemfiles/docs; examples are also updated to the new completion_config call style and document optional resilient defaults.

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