Harden common file helpers against null dereferences, fd 0 mishandling, and relative-path crashes

[Copilot]

This change addresses several severe bugs in the shared file utility layer. The failures were concentrated in src/common/utils/file.c: unsafe error paths in file I/O and a crash path when prepending to files addressed by relative path.

• Safer file_read() / file_write() behavior

  • file_read() now returns NULL on fopen() or allocation failure instead of falling through into invalid buffer access.
  • file_write() now:
    • treats open() failure as -1 instead of misclassifying valid fd 0
    • uses a signed descriptor type
    • rejects short writes
    • closes the descriptor on write failure

• Fix relative-path crash in file_add_line_to_beginning()

  • Handles filenames without / correctly instead of dereferencing a NULL dirname.
  • Replaces unbounded sprintf() with bounded snprintf() when constructing the temporary path.
  • Fails safely if the generated temp path would exceed the fixed buffer.

• Focused regression coverage

  • Extends the host-runnable utility test to include file.c.
  • Adds regression cases for:
    • fd 0 reuse during file_write()
    • prepending to a file referenced by relative path

if ((fd = open(path, O_WRONLY)) == -1)
    return false;

written = write(fd, str, len);
if (written == -1) {
    close(fd);
    return false;
}
if ((size_t)written != len) {
    close(fd);
    return false;
}

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