[libc] Define SSIZE_MAX in limits-macros.h (#201087)

Defined SSIZE_MAX in limits-macros.h to support POSIX compliance.
Applications compiling against LLVM libc require SSIZE_MAX to be defined
in limits.h.

SSIZE_MAX is defined strictly in terms of __PTRDIFF_MAX__, matching
ssize_t defined as __PTRDIFF_TYPE__, ensuring they are always
structurally aligned and compile-time safe.

Added Doxygen comments for SSIZE_MAX.
Added SSIZE_MAX to limits.yaml and added a unit test in limits_test.cpp
to verify the definition.

Assisted-by: Automated tooling, human reviewed.
GitOrigin-RevId: 10d3c6779d9faffed0a1d686ea76ee2cec48bd06