[Flight SQL JDBC] Read Float4 values as double and BigDecimal with correct precision #1406

Describe the bug, including details regarding any error messages, version, and platform.

Describe the bug

ArrowFlightJdbcFloat4VectorAccessor widens the underlying float to double directly. Reading a Float4Vector (SQL REAL/FLOAT4) through ResultSet.getDouble(), ResultSet.getBigDecimal() or ResultSet.getObject(col, Double.class) therefore returns the binary approximation of the float, not the decimal value the float stands for.

• getDouble() is implemented as (double) getFloat().
• getBigDecimal() and getBigDecimal(int scale) use BigDecimal.valueOf(float), which widens to double first.
• getObject(Double.class) goes through getDouble() in ArrowFlightJdbcAccessor.getObject(Class).
• getString() already uses Float.toString(value), so the same cell gives different results depending on the getter:

Result for a 0.1f cell

• getString(): "0.1"
• getDouble(): 0.10000000149011612
• getBigDecimal(): 0.10000000149011612

To Reproduce

try (Float4Vector v = new Float4Vector("f", allocator)) {
  v.setSafe(0, 0.1f);
  v.setValueCount(1);
  ArrowFlightJdbcFloat4VectorAccessor acc =
      new ArrowFlightJdbcFloat4VectorAccessor(v, () -> 0, (wasNull) -> {});
  acc.getString();      // "0.1"
  acc.getDouble();      // 0.10000000149011612  (expected 0.1)
  acc.getBigDecimal();  // 0.10000000149011612  (expected 0.1)
}

The same happens through a real connection with a query returning a REAL column containing 0.1, 1.1, 123.456 and similar values.

Expected behaviour
For a finite float, getDouble(), getBigDecimal() and getObject(Double.class) should return the decimal value the float represents. That is the shortest decimal that uniquely identifies the float, the same one Float.toString yields. For 0.1f that is 0.1, which is what getString() already returns. Other JDBC drivers show this behaviour for REAL columns.

Proposed fix

Convert through the float's decimal representation:
public double getDouble() {
  return Double.parseDouble(Float.toString(this.getFloat()));
}

public BigDecimal getBigDecimal() throws SQLException {
  final float value = this.getFloat();
  if (Float.isInfinite(value) || Float.isNaN(value)) {
    throw new SQLException("BigDecimal doesn't support Infinite/NaN.");
  }
  return this.wasNull ? null : new BigDecimal(Float.toString(value));
}

getBigDecimal(int scale) gets the same change, followed by .setScale(scale, RoundingMode.HALF_UP).