fix(fileutils): prevent stack buffer overflow in AbsPath

[Rakdos8]

Problem

AbsPath (POSIX path, used by CcpGetAbsolutePath) wrote into char[PATH_MAX] buffers with no bounds checking.
getcwd(src, …) followed by strcat(src, path) overflows the stack when cwd + path exceeds PATH_MAX (1024 on
macOS); src[len] = '/'; src[len+1] = 0; writes out of bounds when len == PATH_MAX-1; and strcpy(src, path)
(absolute path) plus strcpy(realPath, path) (on getcwd failure) are unbounded too. This is reachable on macOS (a
primary CI target) via CcpGetAbsolutePath, so it is a high-severity stack overflow driven entirely by path length.

Fix

AbsPath now receives the output buffer size (size_t realPathSize) — there is a single internal caller. src is
built with a bounded snprintf and the function returns an empty string when the combined path does not fit. The
getcwd failure path uses strncpy_s(…, _TRUNCATE) instead of strcpy, and there is an explicit guard before
writing into realPath. CcpGetAbsolutePath drops a dead line (a strcpy_s that was immediately overwritten) and
its unused variable, and now passes sizeof(buffer). Added #include <cstdio> in the POSIX block. Contract: when a
path is too long to fit, the function returns an empty string instead of overflowing.

Tests

Added two regression tests in tests/CcpFileUtils.cpp (GoogleTest, #if !_WIN32):
CcpGetAbsolutePathHandlesOverlongRelativePathSafely and CcpGetAbsolutePathHandlesOverlongAbsolutePathSafely, both
using 64 KB paths and expecting an empty result with no crash. Existing path-resolution tests are unaffected. Reviewed
manually; not compiled locally as the vcpkg toolchain is unavailable in this environment.

Scope

POSIX branch only — the Windows path uses GetFullPathNameW and is unchanged. No public API change.