fix: preserve aggregate result identity during exchange reuse

[sunchao]

Which issue does this PR close?

No issue is automatically closed. This is a standalone aggregate identity fix with a regression added below.

Rationale for this change

Different aggregate result projections can currently have the same Comet plan identity. For example, an exchange above COUNT(*) + 1 can be considered reusable for COUNT(*) - 1, even though the outputs differ. Distinct aggregates also need Spark's original result attributes to canonicalize equivalent plans correctly.

What changes are included in this PR?

• Preserve Spark's original aggregateAttributes in Comet hash and object-hash aggregate conversion.
• Include those attributes in canonicalization and produced attributes, and include aggregate attributes and result expressions in equality and hashing.
• Add four cases to CometAggregateSuite: count, distinct count, distinct count plus sum, and collect-set size. They distinguish different result projections while requiring equivalent projections with fresh aliases/IDs to remain reusable.

Only the aggregate implementation and its existing test suite change. No join feature, native operator, or dependency changes are included.

How are these changes tested?

Draft: JVM validation is blocked, so no passing JVM test result is claimed.

• Built the native library from public main 5baa6b03; a subsequent locked build also passed.
• Spotless checks on the final public patch and git diff --check passed.
• Attempted the focused public Spark 4.0 test run below. Maven dependency resolution stalled on a configured transitive repository before Spark compilation or these tests ran. Repository configuration was left unchanged.

./mvnw test -Pspark-4.0 -Dtest=none \
  -Dsuites=org.apache.comet.exec.CometAggregateSuite \
  '-Dtests=aggregate canonicalization preserves result expressions and equivalent reuse'

The new regressions and affected aggregate tests still need runtime validation before this draft is ready to merge.