fix(issue): reject invalid list output flags

issue list and incident list silently fall back to default rendering for unsupported --output or --output-format values. Use the existing cmdutils.NewEnumValue helper, preserving accepted values, defaults, short flags, and mutual exclusion. Add regression coverage for invalid and empty values on both commands.

Resolves #8547 (closed)

This replaces !3940 (closed) with an identical diff from the community fork so CI can run outside the personal fork. The original MR remains linked for context.

Verification: go test ./internal/commands/issuable/list -count=1 and go test -race ./internal/commands/issuable/list -count=1 passed locally with Go 1.27.1 on macOS arm64. gofmt -d and git diff --check were clean. Community-fork CI: #2864266911.