Preserve trailing newline in block chomping keep indicator (+)

[Tyagiquamar]

Problem

When emitting block scalars with keep chomping indicator (|+), yaml-cpp was deducting 1 from the emitted size for multiline scalars, dropping the trailing newline from the emitted output (#1502).

Solution

• Updated src/emitterutils.cpp to emit the full scalar length for |+ (Keep chomping) without stripping the trailing newline.
• Extended LiteralWithAndWithoutTrailingEmptyLines in test/integration/emitter_test.cpp to verify exact trailing newline preservation with block chomping keep indicator.

Validation

• Built and verified test suite in Linux container with GCC 14:
  • 100% of yaml-cpp unit tests pass.