Preserve floating-point JSON literals as BSON doubles

[IamPritamAcharya]

Description

Fixes an issue where floating-point JSON literals with an integral
numeric value were encoded as integer BSON types.

For example, 3.0 was previously encoded as INT32 because trailing
zeros were stripped before determining the BSON numeric type.

This change preserves the original floating-point notation when
encoding JSON numbers:

• 3 → INT32
• 3.0 → DOUBLE
• 15.9 → DOUBLE
• 1.0E20 → DOUBLE
• -0.0 → DOUBLE

The existing BigDecimal behavior is preserved for values that should
remain DECIMAL128.

Tests

• Added regression coverage for decimal and exponent notation.
• Added signed-zero coverage.
• Added out-of-range floating-point coverage.
• ./gradlew spotlessApply :bson-kotlinx:check passes.

Related to #2031