Structured generation schema invalid when using Array<@Generable type>: missing $defs for referenced item type (v0.7.0)

[ajason]

Summary
When using guided generation with session.respond(to: generating:), schemas containing Array may produce invalid JSON Schema for OpenAI-compatible backends.

The emitted schema contains $ref (e.g. #/$defs/MyModule.PromptRefineEdit) inside array items, but the corresponding $defs entry is missing, causing HTTP 400 invalid_json_schema.

Minimal Repro

import AnyLanguageModel

@Generable
struct Edit {
  let old: String
  let new: String
}

@Generable
struct Result {
  let analysis: String
  let edits: [Edit]
}

let session = LanguageModelSession(model: model)
let _ = try await session.respond(
  to: "Return a valid object",
  generating: Result.self
)

Actual Result
Server rejects request schema:

code: invalid_json_schema
param: text.format.schema
message includes:
reference to component '#/$defs/.Edit' which was not found in the schema
Expected Result
Schema should include all referenced definitions in $defs, including element types referenced by arrays.

Suspected Cause
Array: Generable schema path appears to keep items as $ref but drops element schema defs, so refs become dangling.

Additional Context
Locally patched behavior by preserving elementSchema.defs in Array.generationSchema, which fixed the issue in my app.

[ajason]

I’d like to add an important follow-up finding: there appears to be a second bug beyond the Array schema dependency issue.

Additional finding
Even when PromptRefineResult.generationSchema correctly contains $defs (including DebugTool.ToolRefineEdit), requests can still fail with:

invalid_json_schema
missing #/$defs/DebugTool.ToolRefineEdit
Why this still happens
In the OpenAI strict path, schema conversion appears to do:

GenerationSchema -> JSONSchema -> JSONValue

During this roundtrip, $defs is dropped, and the resulting schema can become only:

{"$ref":"#/$defs/DebugTool.PromptRefineResult"}

So refs remain, but definitions are lost.

Please consider fixing both layers:

Schema generation layer
Ensure Array<Element: Generable> preserves elementSchema.defs.

Provider conversion layer (OpenAI strict mode)
Ensure strict-mode schema transformation does not lose $defs / referenced components.

[mattt]

@ajason Thanks for reporting. Hoping that #118 will resolve these issues. I'll be sure to add explicit test cases to make sure they do.