fix(site): preserve autofill values during initial parameter sync

When the server has not yet evaluated a parameter (param.value.valid is
false), its value is meaningless — it only reflects template defaults,
not autofill or build parameter values. Previously, useSyncFormParameters
would overwrite the form with these empty server values, clearing
autofilled values like '12345' before the server had a chance to process
them.

Use param.value.valid as the discriminator: when false, preserve the
form's existing value; when true, respect the server's evaluated value
(even if empty, since that represents an intentional dynamic
recalculation).

Fixes coder/internal#1154

site/src/modules/hooks/useSyncFormParameters.ts

@@ -16,8 +16,9 @@ export function useSyncFormParameters({
 	formValues,
 	setFieldValue,
 }: UseSyncFormParametersProps) {
-	// Form values only needs to be updated when parameters change
-	// Keep track of form values in a ref to avoid unnecessary updates to rich_parameter_values
+	// Form values only needs to be updated when parameters change.
+	// Keep track of form values in a ref to avoid unnecessary
+	// updates to rich_parameter_values.
 	const formValuesRef = useRef(formValues);
 
 	useEffect(() => {
@@ -27,16 +28,28 @@ export function useSyncFormParameters({
 	useEffect(() => {
 		if (!parameters) return;
 		const currentFormValues = formValuesRef.current;
-
-		const newParameterValues = parameters.map((param) => ({
-			name: param.name,
-			value: param.value.valid ? param.value.value : "",
-		}));
-
 		const currentFormValuesMap = new Map(
 			currentFormValues.map((value) => [value.name, value.value]),
 		);
 
+		const newParameterValues = parameters.map((param) => {
+			// When the server has not evaluated this parameter yet
+			// (valid=false), its value is meaningless — preserve
+			// whatever the form already holds (e.g. autofill or
+			// previous build values). When valid=true, the server
+			// has intentionally set this value and we respect it.
+			if (!param.value.valid) {
+				const currentValue = currentFormValuesMap.get(param.name);
+				if (currentValue) {
+					return { name: param.name, value: currentValue };
+				}
+			}
+			return {
+				name: param.name,
+				value: param.value.valid ? param.value.value : "",
+			};
+		});
+
 		const isChanged =
 			currentFormValues.length !== newParameterValues.length ||
 			newParameterValues.some(