fix: rewrite StepAction generator with pyyaml

The text-based generator broke when tkn task sign mangled the Task
YAML (reordered fields, changed indentation, stripped quotes). The
new generator uses pyyaml to properly parse the Task structure and
produce clean, valid StepAction YAML regardless of input formatting.

Falls back to uv tool run --with pyyaml if pyyaml isn't installed.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Vincent Demeester <vdemeest@redhat.com>

Author: vdemeester