[BUILD] `iceberg` and `iceberg-test` profiles collide on one build-helper execution id, silently dropping `src-iceberg/test` sources

[LuciferYang]

What happens

Both profiles in the root pom declare a build-helper-maven-plugin execution with the id add-iceberg-test-sources: pom.xml:1833 under iceberg, pom.xml:1884 under iceberg-test. Maven merges plugin executions by id, so when both profiles are active only one <sources> list survives.

help:effective-pom with -Pbackends-clickhouse -Pspark-3.5 -Pspark-ut -Pdelta -Piceberg -Piceberg-test resolves that execution to:

add-iceberg-test-sources
    src-iceberg-spark35/test/scala
    src-iceberg-spark35/test/java

The iceberg profile's four entries (src-iceberg/test/{scala,java} and src-iceberg${iceberg.binary.version}/test/{scala,java}) are gone. No warning is printed; the sources are simply not compiled.

Effect on ClickHouse: the combination cannot build

ClickHouseIcebergHiveTableSupport lives in backends-clickhouse/src-iceberg/test/scala, and both Java tests under src-iceberg-spark35/test/java instantiate it. The class therefore leaves the source path at exactly the moment the tests join it:

$ ./build/mvn -Pbackends-clickhouse -Pspark-3.5 -Pscala-2.13 -Pspark-ut -Pdelta \
      -Piceberg -Piceberg-test -DskipTests test-compile -pl backends-clickhouse
[ERROR] .../src-iceberg-spark35/test/java/.../TestPositionDeletesTableGluten.java:[41,18] cannot find symbol
[ERROR]   symbol:   class ClickHouseIcebergHiveTableSupport

Giving the iceberg-test execution a distinct id (add-iceberg-spark-test-sources) and changing nothing else makes the same command exit 0 and emit both ClickHouseIcebergHiveTableSupport.class and TestPositionDeletesTableGluten.class.

Effect on Velox: harmless today, which is why nobody noticed

Nothing in backends-velox needs both directories at once. src-iceberg/test holds VeloxIcebergSuite and VeloxTPCHIcebergSuite, which run in the jobs that pass -Piceberg without -Piceberg-test (velox_backend_x86.yml:941 and :1013, whose wildcardSuites includes org.apache.gluten), plus a @EnhancedFeaturesTest variant that velox_backend_enhanced.yml covers, a JUnit test surefire picks up, and one helper object. The 52 files under src-iceberg-spark34/test run in the three groups at :817, :821, :825 that pass both profiles.

Suggested fix

Rename the iceberg-test profile's execution so the two lists no longer overwrite each other. Both profiles are always passed together in every in-repo invocation, so the merged list is what the plugin was meant to produce.

Two things a reviewer will want to know about the blast radius. The three x86 groups and the 4.0 test-compile at :1277 would newly compile src-iceberg/test/**, so those suites run twice across the matrix; expect longer jobs rather than new failures. And src-iceberg${iceberg.binary.version}/test is empty in every module, so nothing else appears on the source path.

Only backends-velox and backends-clickhouse hold files under src-iceberg*/test, so the change is confined to those two modules.

This blocks the ClickHouse Iceberg Java test work in #12934.