fix: address code review findings from gateway commands PR

[leggetter]

Summary

Fixes issues identified during code review of #213 (gateway resource management commands):

• Missing API key validation: Added ValidateAPIKey() checks to connection enable, connection disable, connection pause, and connection unpause commands — these were the only gateway commands missing the standard guard
• Structured error handling: Replaced string-matching 404 detection in resolveSourceID, resolveDestinationID, resolveTransformationID, and resolveConnectionID with a proper APIError type and IsNotFoundError() helper
• Consistent success indicators: Extracted a shared SuccessCheck constant in helptext.go and replaced all 26 inline checkmark literals across source, destination, transformation, and connection commands
• Event list output: Added Status: and Connection: labels to the text output in event list for readability
• Redundant nil check: Simplified result.Models == nil || len(result.Models) == 0 to len(result.Models) == 0 in resolveSourceID

Note: The original review flagged disabled=false always being sent in connection list as a bug. Investigation revealed this is intentional — see comments in connection_list.go:88-100.

Closes #217

Test plan

• go build ./... — compiles cleanly
• go vet ./... — no new warnings
• go test ./pkg/... — unit tests pass (pre-existing TestSourceCreateRequiresName failure unrelated)
• go test ./test/acceptance/... — 60+ acceptance tests pass covering sources, destinations, connections, transformations, events, requests, and attempts

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