Perform OpenSSH private key PEM encoding in Rust (#15056)

Move the final PEM wrapping (base64 encoding and BEGIN/END markers) of
OpenSSH-format private keys out of Python and into Rust via the existing
encode_der_data helper. The Python serializer now returns the raw OpenSSH key
bytes. As a side effect the base64 body is now wrapped at 64 characters per
line instead of 76.

Fixes #15055


Claude-Session: https://claude.ai/code/session_019igBue8PBCWCi5PD6FqpNt

Co-authored-by: Claude <noreply@anthropic.com>