Preserve named string enum dependencies in schema conversion

[mattt]

Fixes #146

[Copilot]

Pull request overview

This PR fixes a schema-conversion edge case where named string enums (typically coming from $defs / dependencies) were not preserved as named enum schemas during conversion to FoundationModels.DynamicGenerationSchema, causing reference resolution to fail and leading to silent placeholder outputs (Issue #146).

Changes:

• Update JSONSchema → FoundationModels.DynamicGenerationSchema conversion to emit a named string-enum schema when a string schema with enum values is converted with a name.
• Add a regression test ensuring a referenced, named string-enum dependency can be used to build a FoundationModels.GenerationSchema without throwing.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Sources/AnyLanguageModel/Models/SystemLanguageModel.swift	Preserves named string-enum dependencies by converting them into named anyOf: [String] schemas, enabling reference resolution.
Tests/AnyLanguageModelTests/DynamicSchemaConversionTests.swift	Adds a regression test covering a named string enum dependency referenced from a root schema.

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