fix: skip unresolvable request parameter and body refs instead of panicking

[SAY-5]

When an operation has a parameter or request body $ref that cannot be resolved (e.g. it points at the wrong component section, yielding MismatchedRef), RequestPane::init_schema and ParameterEditor::init_parameters would call .unwrap() and crash the whole TUI; this skips those refs the same way response.rs and body_editor.rs already do. Closes #42.