fix(site): resolve WS/HTTP race condition on workspace parameters page (backport v2.29) (#24249)

Cherry-pick of
https://github.com/coder/coder/commit/17d214b4a4a75a39fc499784dd2e65f81b7cdcf1
onto `release/2.29`.

Original PR: #22556

When the workspace parameters page loads, the WebSocket sends an initial
response with template defaults. For parameters with no default, the
server returns `{valid: false, value: ""}`. On first render,
`useSyncFormParameters` overwrites the form's correctly-autofilled value
with `""`. This fix preserves the current form value when the server
value is `{valid: false}`.

Fixes the consistently failing e2e test `create workspace with default
and required parameters`.

> 🤖 Generated by Coder Agents

Co-authored-by: Kyle Carberry <kyle@coder.com>

site/src/modules/hooks/useSyncFormParameters.ts

@@ -27,16 +27,30 @@ 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 value is not valid (e.g., the initial
+			// WebSocket response before any user input is sent),
+			// preserve the current form value. This prevents the sync
+			// hook from overwriting autofilled values (from the
+			// previous build) with empty strings before the server
+			// has had a chance to process them.
+			if (!param.value.valid) {
+				const existingValue = currentFormValuesMap.get(param.name);
+				if (existingValue !== undefined) {
+					return { name: param.name, value: existingValue };
+				}
+			}
+
+			return {
+				name: param.name,
+				value: param.value.valid ? param.value.value : "",
+			};
+		});
+
 		const isChanged =
 			currentFormValues.length !== newParameterValues.length ||
 			newParameterValues.some(