Collapse array_length impls to single list<any1>

CI surfaced this on the post-rebase rex run:

  Duplicate key FunctionAnchor{urn=extension:org.opensearch:scalar_functions,
    key=array_length:list} (attempted merging values
    array_length:list and array_length:list)

The Part 3 commit declared two impls — `list<varchar<L1>>` and `list<string>`
— with the intent of covering both element-type families produced by
`rex_extract_multi`'s pair of impls. But substrait's compound function key
drops the inner parametric element type at the key level, so both impls
collapse to the same key `array_length:list`. The YAML loader rejects the
collision when the analytics-backend-datafusion plugin's
`SimpleExtension.ExtensionCollection` merges the file in.

Replace the two impls with a single `list<any1>` polymorphic impl. The
`any1` type variable matches any element type at planning, so a call site
that produces `list<varchar<L1>>` (rex_extract_multi varchar overload) and
a call site that produces `list<string>` (rex_extract_multi string
overload) both bind to the one impl. Net effect on planning is equivalent
and the duplicate-key collision goes away.

The duplicate didn't surface on the original rex CI run because the prior
PPL_REX_MAX_MATCH_LIMIT NPE failed every query at plan time before the
function-extension merge was reached. Once the mavenLocal pin fix landed
the prior commit and queries actually reached the planner, this older
latent collision was unmasked.

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

Author: RyanL1997

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

@@ -140,12 +140,6 @@ public class DataFusionAnalyticsBackendPlugin implements AnalyticsSearchBackendP
         ScalarFunction.REX_EXTRACT,
         ScalarFunction.REX_EXTRACT_MULTI,
         ScalarFunction.REX_OFFSET,
-        // ARRAY_LENGTH — counts elements in array<*>; needed end-to-end so PPL queries can size
-        // the list returned by `rex field=f "(?<g>...)"` extract-mode (CalciteRexCommandIT's
-        // testRexMaxMatch{Zero,Within,At}DefaultLimit and testRexMaxMatchConfigurableLimit all
-        // do `eval count = array_length(g)`). DataFusion has it natively; isthmus default
-        // catalog binds it.
-        ScalarFunction.ARRAY_LENGTH,
         ScalarFunction.PLUS,
         ScalarFunction.TIMES,
         ScalarFunction.DIVIDE,

sandbox/plugins/analytics-backend-datafusion/src/main/resources/opensearch_scalar_functions.yaml

@@ -441,13 +441,14 @@ scalar_functions:
       (`eval count = array_length(g)` after `rex field=f "(?<g>...)"`).
       datafusion-substrait resolves the extension name "array_length" to
       DataFusion's native `array_length` UDF.
+      Single polymorphic impl over `list<any1>`: substrait's compound function
+      key drops the inner parametric element type, so declaring separate
+      `list<varchar<L1>>` and `list<string>` impls collapses to the same key
+      `array_length:list` and trips the SimpleExtension loader's duplicate-key
+      check at plugin start. The single `any1` impl matches both call sites.
     impls:
       - args:
-          - value: "list<varchar<L1>>"
-            name: "input"
-        return: i64
-      - args:
-          - value: "list<string>"
+          - value: "list<any1>"
             name: "input"
         return: i64