Implement native function of `bround`

[slfan1989]

Description

Implement Spark-compatible bround() (bankers’ rounding) function

In PR #1327, we introduced support for the round() function following Spark’s semantics (HALF_UP rounding mode).
However, Spark also provides a companion function bround() that performs bankers’ rounding (HALF_EVEN mode), which rounds ties to the nearest even number.

To achieve full compatibility with Spark’s numeric functions, we should implement bround() with the following characteristics:

Expected behavior

Function name: bround(expr, scale)
Rounding mode: HALF_EVEN (bankers’ rounding)
Example:

• bround(2.5) → 2.0
• bround(3.5) → 4.0
• bround(-2.5) → -2.0

Supports: FLOAT, DOUBLE, DECIMAL, INT16/32/64

• Handles negative scales: e.g., bround(123.4, -1) = 120
• Null-safe: should return NULL if input is NULL
• Array and scalar inputs: consistent with current round() implementation

Motivation

• Ensure feature parity with Apache Spark SQL
• Improve compatibility for expressions and queries migrated from Spark
• Reduce fallback occurrences to Spark UDF wrapper