fix: replace unreachable!() with error return in setup wizard

The provider match in step_inference_provider was guarded by
is_known but used unreachable!() as the catch-all. If a new
provider is added to the is_known check without a corresponding
match arm, this would panic at runtime. Return a typed error
instead.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

src/setup/wizard.rs

@@ -605,7 +605,12 @@ impl SetupWizard {
                     "openai" => return self.setup_openai().await,
                     "ollama" => return self.setup_ollama(),
                     "openai_compatible" => return self.setup_openai_compatible().await,
-                    _ => unreachable!(),
+                    _ => {
+                        return Err(SetupError::Config(format!(
+                            "Unhandled provider: {}",
+                            current
+                        )));
+                    }
                 }
             }