fix(site/src/pages/TemplateBuilder): use variable defaults as field values, not just placeholders (#27032)

Variables with defaults showed the default as placeholder text in an
empty field. If the user did not type anything, an empty value was sent
instead of the default. Now the default pre-populates the field value so
it is visible and editable.

Fixes both `BaseTemplateParametersStep` and `ModuleSettingsStep` by
falling back to `defaultPlaceholder(v.default)` instead of `""` when no
user value exists.

Fixes DEVEX-584

> [!NOTE]
> This PR was authored by Coder Agents on behalf of @jeremyruppel.

site/src/pages/TemplateBuilder/BaseTemplateParametersStep.tsx

@@ -109,7 +109,11 @@ export const BaseTemplateParametersStep: FC<
 	};
 
 	const fields: ConfigurationFieldDefinition[] = variables.map((v) =>
-		variableToField(v, values[v.name] ?? "", handleChange),
+		variableToField(
+			v,
+			values[v.name] ?? defaultPlaceholder(v.default) ?? "",
+			handleChange,
+		),
 	);
 
 	return (

site/src/pages/TemplateBuilder/ModuleSettingsStep.tsx

@@ -135,8 +135,11 @@ export const ModuleSettingsStep: FC<ModuleSettingsStepProps> = ({
 					const vars = moduleVariables[mod.id] ?? {};
 
 					const toField = (v: TemplateBuilderModuleVariable) =>
-						variableToField(mod.id, v, vars[v.name] ?? "", (name, val) =>
-							handleChange(mod.id, name, val),
+						variableToField(
+							mod.id,
+							v,
+							vars[v.name] ?? defaultPlaceholder(v.default) ?? "",
+							(name, val) => handleChange(mod.id, name, val),
 						);
 
 					const requiredVars = configurableVars.filter((v) => v.required);