fix(webapp): keep preview deployments enabled when the plan gates it

With the Preview environment gated by plan there is no toggle to submit, so
saving Git settings sent no previewDeploymentsEnabled value and the schema
transformed the absence into false, silently clearing a setting the user could
not see. A hidden input now carries the persisted value through the save.

Pre-existing: the toggle it replaced was rendered unchecked and disabled when
gated, so it submitted nothing either.

Author: samejr

apps/webapp/app/routes/resources.orgs.$organizationSlug.projects.$projectParam.env.$envParam.github.tsx

@@ -989,14 +989,19 @@ export function ConnectedGitHubRepoForm({
                 }}
               />
             ) : (
-              <LinkButton
-                to={billingPath}
-                variant="secondary/small"
-                LeadingIcon={ArrowUpCircleIcon}
-                leadingIconClassName="text-indigo-500"
-              >
-                Upgrade
-              </LinkButton>
+              <>
+                {connectedGitHubRepo.previewDeploymentsEnabled && (
+                  <input type="hidden" name="previewDeploymentsEnabled" value="on" />
+                )}
+                <LinkButton
+                  to={billingPath}
+                  variant="secondary/small"
+                  LeadingIcon={ArrowUpCircleIcon}
+                  leadingIconClassName="text-indigo-500"
+                >
+                  Upgrade
+                </LinkButton>
+              </>
             )
           }
         >