Function parse_step 

pub fn parse_step(value: &Value, source: &str) -> Result<Step, ParseError>

Parse a single YAML entry from a pre_invocation / post_invocation list.

Two YAML shapes (DSL §3.2 + §7):

• String entry — a rule line, taint effect, or plugin call.
  • "require(authenticated)" → Step::Rule
  • "delegation.depth > 2: deny" → Step::Rule
  • "plugin(rate_limiter)" → Step::Plugin ("run(rate_limiter)" is an alias)
  • "taint(PII, session)" → Step::Taint
• Map entry (single-key map) — PDP call with optional reactions.
  • cedar: { action: read, resource: e, on_deny: [...] } → Step::Pdp
  • opa("path"): { on_deny: [...] } → Step::Pdp