why

what changed

test plan

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Summary by cubic

Fixes Zod v3 enum handling for method selection in act() and observe() to prevent v3 runtime errors and keep validation consistent across Zod v3 and v4.

• Bug Fixes
  • Use Object.values(SupportedUnderstudyAction) with z.enum() in inference.ts for act() and observe().
  • Add tests verifying enum validation works in both Zod v3 and v4, including invalid method rejection.
  • Supports Linear STG-1212 by enforcing enum-based method selection in act/observe.