[Analytics Backend / DataFusion] Bind PPL concat() to substrait concat

PPL's `concat(a, b, …)` registers in `PPLFuncImpTable` as
`SqlLibraryOperators.CONCAT_FUNCTION` (operator name `"CONCAT"`), distinct
from the `||` binary string-concat operator
(`SqlStdOperatorTable.CONCAT`) that isthmus' default Calcite→Substrait Sig
table already binds to substrait `concat`. Without an entry for the
function form, isthmus' `RexExpressionConverter.visitCall` runs through
its `callConverters` list, finds no match, and throws
`Unable to convert call CONCAT(string, string)`.

Concretely this surfaced on `testMvfindWithDynamicRegex`, which builds
its regex via `concat('ban', '.*')` and feeds it to `mvfind`. Substrait's
default `functions_string.yaml` already declares `concat` so no YAML
change is needed — only the Sig bridge.

# Pass-rate (CalciteArrayFunctionIT, force-routed)

  * Before: 42/60.
  * After:  43/60.

  Newly passing: testMvfindWithDynamicRegex.

Signed-off-by: Kai Huang <ahkcs@amazon.com>

Author: ahkcs

sandbox/plugins/analytics-backend-datafusion/src/main/java/org/opensearch/be/datafusion/DataFusionFragmentConvertor.java

@@ -113,6 +113,13 @@ public class DataFusionFragmentConvertor implements FragmentConvertor {
         FunctionMappings.s(SqlStdOperatorTable.RAND, "random"),
         FunctionMappings.s(SqlLibraryOperators.LOG, "logb"),
         FunctionMappings.s(SignumFunction.FUNCTION, SignumFunction.NAME),
+        // PPL `concat()` (registered in PPLFuncImpTable as SqlLibraryOperators.CONCAT_FUNCTION,
+        // operator name "CONCAT") — distinct from `||` aka SqlStdOperatorTable.CONCAT, which
+        // isthmus' default catalog already binds to substrait `concat`. Without this bridge,
+        // any PPL eval using `concat(...)` (e.g. dynamic-pattern flows like
+        // `mvfind(arr, concat('ban', '.*'))`) fails substrait conversion with
+        // "Unable to convert call CONCAT(string, string)".
+        FunctionMappings.s(SqlLibraryOperators.CONCAT_FUNCTION, "concat"),
         // Array S0 ladder — see DataFusionAnalyticsBackendPlugin.STANDARD_PROJECT_OPS /
         // ARRAY_RETURNING_PROJECT_OPS for the capability registration. ARRAY_LENGTH /
         // ARRAY_SLICE / ARRAY_DISTINCT pass through under their Calcite-stdlib names