Return errors from schema generation instead of panicking

jsonSchemaFor panicked on unsupported types, which meant
FunctionTool, NewOutputType, and RunTyped would crash the
process during setup rather than returning a normal error.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/agent/handoff.go

@@ -45,7 +45,7 @@ type (
 )
 
 var (
-	handoffParamsSchema = jsonSchemaFor[handoffParams]()
+	handoffParamsSchema = mustJSONSchemaFor[handoffParams]()
 )
 
 func HandoffTo(agent *Agent, opts ...HandoffOption) *Handoff {