Mark SetModelOptions non_exhaustive

Adding the auto_tier field made SetModelOptions source-breaking for
external crates constructing it as a full struct literal. Every other
options struct in this file (including CapiSessionOptions, extended the
same way in #2437) already carries #[non_exhaustive], so apply the same
convention here.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

rust/src/types.rs

@@ -4781,6 +4781,7 @@ impl LogOptions {
 ///
 /// Pass `None` to `set_model` to switch model without any overrides.
 #[derive(Debug, Clone, Default)]
+#[non_exhaustive]
 pub struct SetModelOptions {
     /// Reasoning effort for the new model (e.g. `"low"`, `"medium"`,
     /// `"high"`, `"xhigh"`, `"max"`).