[Feature](agg) Add aggregate function entropy

[Copilot]

Implements Shannon Entropy (base-2) as a new aggregate function, supporting single-column numeric/string types and multi-column generic inputs via serialized composite keys.

BE (aggregate_function_entropy.h/.cpp)

• AggregateFunctionEntropyData<Value, Hash>: frequency map + total_count field tracked incrementally so get_result() is a single pass (original two-pass design iterated the map twice).
• Three data specializations:
  • SingleNumericData<T>: direct typed value key
  • SingleStringData: XXH128 hash of raw bytes
  • GenericData: XXH128 hash of arena-serialized concatenation of all columns (multi-arg support)
• AggregateFunctionEntropy<Data>: marked final, inherits VarargsExpression + NullableAggregateFunction; registered via register_function_both.

FE (Entropy.java)

NullableAggregateFunction with varArgs(AnyDataType) → DoubleType; wired into BuiltinAggregateFunctions and AggregateFunctionVisitor.visitEntropy.

Test infrastructure fixes

• agg_function_test.h: Guards deserialize_and_merge_from_column_range with block.rows() != 0 to prevent size_t underflow on empty input (was passing 0 - 1 = SIZE_MAX as end index).
• column_helper.h: Extends two-column create_block to create_block<T1, T2 = T1> so mixed-type blocks (e.g. int + string) can be constructed in unit tests.

Tests

Unit tests (agg_entropy_test.cpp) cover numeric, string, multi-column generic, nullable (NULLs skipped), and empty-input cases via serialize/deserialize/merge round-trips. Regression suite (aggregate_function_entropy.groovy) exercises all SQL types including complex types, GROUP BY, and window functions.

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[Thearas]

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