Function resolve_mode

core/src/llm/structured.rs:890–902

Resolve the requested mode against the provider's native capability. `Auto`/`Tool` always resolve to forced `Tool` mode — the most reliable cross-provider strategy (the synthetic `emit_*` tool is made mandatory via the provider's `tool_choice`). `Strict`/`Json` use native `response_format` only when the provider reports [`NativeStructuredSupport::JsonSchema`]; otherwise they fall back to forced `

(requested: StructuredMode, support: NativeStructuredSupport)

/// otherwise they fall back to forced `Tool` mode rather than silently
/// degrading to unconstrained text.
fn resolve_mode(requested: StructuredMode, support: NativeStructuredSupport) -> StructuredMode {
    match requested {
        StructuredMode::Prompt => StructuredMode::Prompt,
        StructuredMode::Strict if support == NativeStructuredSupport::JsonSchema => {
            StructuredMode::Strict
        }
        StructuredMode::Json if support == NativeStructuredSupport::JsonSchema => {
            StructuredMode::Json
        }
        // Auto, Tool, or Strict/Json on a provider without json_schema support.
        _ => StructuredMode::Tool,
    }
}

/// Build the provider directive for an already-resolved mode.
fn build_directive(req: &StructuredRequest, mode: StructuredMode) -> StructuredDirective {