fix(isthmus): route native-image metadata queries through reflective provider

[nielspardon]

Problem

Queries like the following crash in the isthmus native image (#251):

./isthmus -c 'CREATE TABLE foo(a VARCHAR);' "SELECT a FROM foo GROUP BY a"
./isthmus -c 'CREATE TABLE foo(a VARCHAR);' "SELECT DISTINCT a FROM foo"

A grouping-only GROUP BY (and SELECT DISTINCT) goes through
RelBuilder.aggregate_ → alreadyUnique() → RelMetadataQuery.areColumnsUnique().
Calcite's default RelMetadataQuery obtains its handlers from
JaninoRelMetadataProvider, which compiles a handler class at runtime with Janino.
Runtime code generation is unavailable in a GraalVM native image (closed-world,
ahead-of-time compiled), so the query fails:

java.lang.AssertionError: Required type "Ljava/lang/AssertionError;" not found
    at org.codehaus.janino.IClassLoader.requireType
    at org.apache.calcite.rel.metadata.JaninoRelMetadataProvider.compile

An aggregate with a function (count(...)) skips the uniqueness check, so the
existing smoke test never exercised the broken path — which is why this stayed
uncaught.

Fix

Calcite ships ProxyingMetadataHandlerProvider, a reflection-based (dynamic-proxy)
alternative to the Janino code generator. Dynamic proxies are supported in native
images once their interfaces are registered. SqlConverterBase already used this
provider — but only the SQL-expression path used that cluster. The SQL-query path
(SubstraitSqlToCalcite) and the Substrait→Calcite path
(ConverterProvider.getRelBuilder, SubstraitRelNodeConverter.convert) built
clusters with no metadata supplier and fell back to Janino.

• Extract the setup into Utils.useReflectiveMetadataProvider(RelOptCluster) and
  apply it to every cluster / RelBuilder used for Calcite conversion (removing the
  duplicated inline lambda in SqlConverterBase).
• The reflective provider proxies the bare BuiltInMetadata$* interfaces, not
  just the $Handler sub-interfaces already present, so register those in
  reachability-metadata.json (the safe superset — unused proxy registrations are
  harmless).
• Add grouping-only GROUP BY and SELECT DISTINCT cases to smoke.sh so the
  native image exercises the uniqueness path in CI.

Closes #251

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