syscalls: beef up write(2) and pwrite(2) tests

[copybara-service]

syscalls: beef up write(2) and pwrite(2) tests

Fixes #2370

Add comprehensive tests for write(2), pwrite(2), and writev(2) syscalls
to improve coverage as requested in the issue.

New tests include:

• Bad buffer states (EFAULT): nullptr buffer, bad iov_base address
• Bad file descriptors (EBADF): closed fd, negative fd, read-only fd, O_PATH fd
• Offset behavior: verify offset increments, pwrite doesn't change offset
• EOF handling: write extends past EOF, pwrite creates holes
• Pipes: write to pipe, pwrite returns ESPIPE, write to read end fails
• Symlinks: write through symlink, O_NOFOLLOW returns ELOOP
• Special files: /dev/null and /dev/zero
• writev edge cases: empty iovec, zero-length entries

Also adds similar tests to pwrite64.cc for consistency.

FUTURE_COPYBARA_INTEGRATE_REVIEW=#12426 from 0xzre:write-syscall-tests 4f4666f