Default plugins.ppl.rex.max_match.limit=10 on the unified query path

The PPL `rex` command's AstBuilder reads `Settings.Key.PPL_REX_MAX_MATCH_LIMIT`
unconditionally and unboxes the result to `int`:

  int maxMatchLimit =
      (settings != null) ? settings.getSettingValue(...) : 10;

The `(settings != null)` guard only protects against the Settings instance
being null — not against `getSettingValue` returning null for a key that the
caller never registered. On the unified query path, `UnifiedQueryContext`
builds its `Settings` map with only a small whitelist of keys (e.g.
`QUERY_SIZE_LIMIT`, `CALCITE_ENGINE_ENABLED` per #5413). For any unregistered
key, `getSettingValue` returns null, and the auto-unbox NPEs the planner
before any operator-level capability check runs. Every `rex` query through
`/_analytics/ppl` (the analytics-engine route's REST front-end) hits this
NPE today.

Default `PPL_REX_MAX_MATCH_LIMIT=10` in `buildSettings()` so unified-path
behavior matches the cluster-side default registered by
`OpenSearchSettings.PPL_REX_MAX_MATCH_LIMIT_SETTING` — making the v2 path and
the analytics-engine path agree on the same fallback value when neither has
an explicit cluster override. Mirrors the precedent Kai introduced for
`CALCITE_ENGINE_ENABLED` in #5413.

Companion to the OpenSearch core PR onboarding PPL `rex mode=sed` to the
analytics-engine route via DataFusion (Part 1 — sed-mode bridge only;
extract-mode Rust UDFs deferred to Part 2).

Signed-off-by: Jialiang Liang <jiallian@amazon.com>

Author: RyanL1997

api/src/main/java/org/opensearch/sql/api/UnifiedQueryContext.java

@@ -7,6 +7,7 @@
 
 import static org.opensearch.sql.common.setting.Settings.Key.CALCITE_ENGINE_ENABLED;
 import static org.opensearch.sql.common.setting.Settings.Key.PPL_JOIN_SUBSEARCH_MAXOUT;
+import static org.opensearch.sql.common.setting.Settings.Key.PPL_REX_MAX_MATCH_LIMIT;
 import static org.opensearch.sql.common.setting.Settings.Key.PPL_SUBSEARCH_MAXOUT;
 import static org.opensearch.sql.common.setting.Settings.Key.QUERY_SIZE_LIMIT;
 
@@ -123,14 +124,22 @@ public static class Builder {
      * org.opensearch.sql.api.parser.PPLQueryParser} reuses the v2 {@code AstBuilder}, which gates
      * Calcite-only commands (e.g. {@code visitTableCommand}) on this setting; without the default,
      * those commands fail at parse time even when the cluster setting is true.
+     *
+     * <p>{@link Settings.Key#PPL_REX_MAX_MATCH_LIMIT} defaults to {@code 10} here because
+     * {@code AstBuilder.visitRexCommand} reads it unconditionally and unboxes to {@code int} —
+     * a {@code null} return from {@code getSettingValue} NPEs the planner before any operator-
+     * level capability check runs. The value mirrors the cluster-side default of {@code 10}
+     * registered by {@code OpenSearchSettings.PPL_REX_MAX_MATCH_LIMIT_SETTING}, so unified-path
+     * behavior matches v2-path behavior when neither has an explicit cluster override.
      */
     private final Map<Settings.Key, Object> settings =
         new HashMap<Settings.Key, Object>(
             Map.of(
                 QUERY_SIZE_LIMIT, SysLimit.DEFAULT.querySizeLimit(),
                 PPL_SUBSEARCH_MAXOUT, SysLimit.DEFAULT.subsearchLimit(),
                 PPL_JOIN_SUBSEARCH_MAXOUT, SysLimit.DEFAULT.joinSubsearchLimit(),
-                CALCITE_ENGINE_ENABLED, true));
+                CALCITE_ENGINE_ENABLED, true,
+                PPL_REX_MAX_MATCH_LIMIT, 10));
 
     /**
      * Sets the query language frontend to be used.