fix(issue): reject invalid list output flags

Mahidul Haquerequested to merge
Kxrma47/cli:kxrma47/8547-validate-list-output into main

Description

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

Related issue

Resolves #8547 (closed)

Verification

  • go test ./internal/commands/issuable/list -count=1 passed locally with Go 1.27.1 on macOS arm64.
  • gofmt -d and git diff --check were clean.
  • The fork pipeline failed before running jobs because GitLab reports this account is unverified; this is not a test failure.

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