chore: improve readonly prop messaging (#9901)

Author: trueadm

.changeset/large-turkeys-deny.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: improve readonly prop messaging

packages/svelte/src/internal/client/proxy/readonly.js

@@ -42,7 +42,7 @@ export function readonly(value) {
  */
 const readonly_error = (_, prop) => {
 	throw new Error(
-		`Non-bound props cannot be mutated — use \`bind:<prop>={...}\` to make \`${prop}\` settable. Fallback values can never be mutated.`
+		`Non-bound props cannot be mutated — to make the \`${prop}\` settable, ensure the object it is used within is bound as a prop \`bind:<prop>={...}\`. Fallback values can never be mutated.`
 	);
 };
 

packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/_config.js

@@ -15,5 +15,5 @@ export default test({
 	},
 
 	runtime_error:
-		'Non-bound props cannot be mutated — use `bind:<prop>={...}` to make `count` settable. Fallback values can never be mutated.'
+		'Non-bound props cannot be mutated — to make the `count` settable, ensure the object it is used within is bound as a prop `bind:<prop>={...}`. Fallback values can never be mutated.'
 });

packages/svelte/tests/runtime-runes/samples/proxy-prop-readonly/_config.js

@@ -15,5 +15,5 @@ export default test({
 	},
 
 	runtime_error:
-		'Non-bound props cannot be mutated — use `bind:<prop>={...}` to make `count` settable. Fallback values can never be mutated.'
+		'Non-bound props cannot be mutated — to make the `count` settable, ensure the object it is used within is bound as a prop `bind:<prop>={...}`. Fallback values can never be mutated.'
 });