fix: address review on query options consistency

- federated: absorb SHOW VARIABLES with a WHERE/LIKE filter or a
  non-identifier remainder as an empty table; only a bare or
  dotted-identifier remainder falls back to the parser so named variables
  like query.parallelism reach the real handler (restores Connector/J 5.1
  loadServerVariables behavior).
- datanode: report engine_context query-option failures as
  InvalidQueryContext instead of NewPlanDecoder.
- http: drop the redundant Option around the always-present timeout layer.
- session: split the query.parallelism 1024 cap to public entry points
  (SQL SET, HTTP hints) and keep wire decoding at u16::MAX so a legacy
  frontend's nonzero value such as 2048 still restores.

Signed-off-by: discord9 <55937128+discord9@users.noreply.github.com>

src/query/src/sql.rs

@@ -855,7 +855,12 @@ pub fn show_query_option(
         if name.to_ascii_lowercase().starts_with("query.")
             || name.to_ascii_lowercase().starts_with("datafusion.")
         {
-            session::query_options::parse_query_option(&name, "").map_err(|error| {
+            session::query_options::parse_query_option(
+                &name,
+                "",
+                session::query_options::MAX_PUBLIC_PARALLELISM,
+            )
+            .map_err(|error| {
                 error::InvalidQueryContextExtensionSnafu {
                     reason: error.to_string(),
                 }