test: only assert genuinely invalid read-preference values at request ingresses (#9576)

The flow and nested-MergeScan ingress tests hardcoded "follower" among
the read-preference values that must be rejected, coupling them to the
current leader-only ReadPreference enum. Assert only genuinely invalid
values there, matching the wire-values test which already limits itself
to ["", "unknown"]. This keeps the ingress tests valid for builds that
extend ReadPreference with more variants.

Signed-off-by: Ning Sun <sunning@greptime.com>

tests-integration/src/cluster.rs

@@ -1170,7 +1170,7 @@ mod tests {
             response.unwrap_err().status_code()
         );
 
-        for value in ["", "unknown", "follower"] {
+        for value in ["", "unknown"] {
             let mut query_context =
                 api::v1::QueryContext::from(session::context::QueryContext::arc().as_ref());
             query_context.extensions.insert(

tests-integration/src/tests/nested_merge_scan_capability_test.rs

@@ -158,7 +158,7 @@ async fn test_nested_merge_scan_capability_rejects_internal_read_preference() {
         .map(|id| (*id, rpc_requests(&cluster, *id)))
         .collect::<BTreeMap<_, _>>();
 
-    for value in ["", "invalid", "follower"] {
+    for value in ["", "invalid"] {
         let mut header = region_request_header(&query_ctx);
         header
             .query_context