feat: add Dictionary support to @Generable macro

[kylebrowning]

Summary

@Generable currently doesn't support Dictionary property types. Properties like [String: String] cause compilation errors because Dictionary doesn't conform to Generable or ConvertibleFromGeneratedContent.

This PR adds full Dictionary support across all three layers:

Schema layer (GenerationSchema.swift)

• Added .dictionary(DictionaryNode) case to GenerationSchema.Node
• Added DictionaryNode struct with description and values fields
• Encodes as JSON Schema {"type": "object", "additionalProperties": <value_schema>}
• Decodes by distinguishing objects (with properties) from dictionaries (with additionalProperties)

Protocol layer (Generable.swift)

• Added extension Dictionary: Generable where Key == String, Value: Generable
• Returns .dictionary(values: Value.schema) for the schema

Content conversion layer

• Added Dictionary: ConvertibleFromGeneratedContent — extracts key-value pairs from .structure content
• Added Dictionary: ConvertibleToGeneratedContent — converts entries to .structure content

Macro layer

No changes needed — the existing macro already had isDictionaryType() and extractDictionaryTypes() helpers. With Dictionary now conforming to Generable, the existing code paths work correctly.

Example

@Generable
struct TaskAction {
    var actionType: String
    var confidence: Double
    var parameters: [String: String]  // Now works!
}

Test plan

• swift build passes on the Conduit repo
• Verify @Generable structs with [String: String] properties compile and generate correct schemas
• Verify round-trip: generated content → Dictionary → generated content

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Carry value defs when building dictionary schema

When Value.generationSchema.root is a $ref (for example, Value is another @Generable type), this code stores that ref in additionalProperties but drops Value.generationSchema.defs by returning GenerationSchema.primitive(...). GenerationSchema.Property.buildNode later only merges schema.defs, so schemas for [String: Value] can emit dangling $refs with missing definitions, which breaks nested dictionary value typing in structured generation/tool schemas.

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[chatgpt-codex-connector]

Keep Anthropic tool schema compatibility for dictionaries

This new encoding writes additionalProperties as a nested schema object, but Anthropic conversion currently reads additionalProperties only as Bool (convertToInputSchema and convertToPropertySchema in AnthropicProvider+Helpers.swift), so dictionary constraints are silently dropped to an unconstrained object for Anthropic tool calls. As a result, dictionary-typed tool arguments are not validated to the declared value type and can produce downstream conversion failures.

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