fix: avoid eval in fish completion requests

[puneetdixit200]

Summary

• generate fish completions through yq-specific post-processing so the completion request is invoked as a fish argument list instead of an eval string
• keep YQ_ACTIVE_HELP=0 exported locally for the request
• add regression coverage for the generated fish script

Fixes #2503

Testing

• GOCACHE=/tmp/yq-2503-gocache GOMODCACHE=/tmp/yq-2503-gomodcache go test ./cmd -run TestFishCompletionDoesNotEvalCompletionRequest -count=1
• GOCACHE=/tmp/yq-2503-gocache GOMODCACHE=/tmp/yq-2503-gomodcache go test ./cmd -count=1
• GOCACHE=/tmp/yq-2503-gocache GOMODCACHE=/tmp/yq-2503-gomodcache go test ./... -count=1
• PATH=/tmp/yq-2503-gobin:$PATH GOCACHE=/tmp/yq-2503-gocache GOMODCACHE=/tmp/yq-2503-gomodcache ./scripts/check.sh
• GOCACHE=/tmp/yq-2503-gocache GOMODCACHE=/tmp/yq-2503-gomodcache go run . completion fish | rg -n 'set -l results \\(\\$requestComp 2> /dev/null\\)|eval \\$requestComp|set -lx YQ_ACTIVE_HELP 0'\n- git diff --check