Fix fish custom completion arguments

[puneetdixit200]

What type of PR is this?

• bug

What this PR does / why we need it:

• Updates the generated fish completion script so it only forwards the current token when that token starts with -.
• Makes fish match the bash and zsh behavior for dynamic completions by omitting partial positional tokens from the command invocation.
• Adds completion coverage for the fish script behavior and nested custom subcommand completion dispatch.

Which issue(s) this PR fixes:

Fixes #2333

Special notes for your reviewer:

The reported callback checks NArg() before printing completions. In fish, the partial positional token was being forwarded before --generate-shell-completion, so callbacks like that saw an argument and returned without output.

Testing

• GOMODCACHE=$PWD/.gomodcache GOCACHE=$PWD/.gocache go test ./... -run 'TestCompletionFishOmitsPositionalTokenFromDynamicCompletion|TestCompletionSubcommandCustomShellComplete|TestCompletionFishFormat' -count=1\n- GOMODCACHE=$PWD/.gomodcache GOCACHE=$PWD/.gocache go test ./...\n- GOMODCACHE=$PWD/.gomodcache GOCACHE=$PWD/.gocache make vet\n- GOMODCACHE=$PWD/.gomodcache GOCACHE=$PWD/.gocache make test\n- GOMODCACHE=$PWD/.gomodcache GOCACHE=$PWD/.gocache make check-binary-size\n- GOMODCACHE=$PWD/.gomodcache GOCACHE=$PWD/.gocache make generate\n- PATH=$PWD/.local/bin:$PATH GOMODCACHE=/tmp/urfave-cli-gomodcache GOCACHE=/tmp/urfave-cli-gocache make lint\n- PATH=$PWD/.local/bin:$PATH GOMODCACHE=/tmp/urfave-cli-gomodcache GOCACHE=/tmp/urfave-cli-gocache make gfmrun\n- PATH=$PWD/.local/bin:$PATH GOMODCACHE=/tmp/urfave-cli-gomodcache GOCACHE=/tmp/urfave-cli-gocache make v3diff\n- PATH=$PWD/.local/bin:$PATH GOMODCACHE=/tmp/urfave-cli-gomodcache GOCACHE=/tmp/urfave-cli-gocache make diffcheck\n- PATH=$PWD/.local/bin:$PATH GOMODCACHE=/tmp/urfave-cli-gomodcache GOCACHE=/tmp/urfave-cli-gocache golangci-lint run --new-from-rev=upstream/main\n- git diff --check\n\nNote: on macOS arm64, make ensure-gfmrun could not download the gfmrun v1.3.0 release asset because GitHub returned 404, so I installed gfmrun v1.3.0 from source locally before running make gfmrun.\n\n## Release Notes\n\nrelease-note\nFixed fish shell completion for custom subcommand completions.\n

[TLINDEN]

In #2333 I am using bash completion, are you sure, this PR would really fix this too?

[puneetdixit200]

Yes. The runtime change is fish-only because bash already omits the partial positional token before invoking --generate-shell-completion; the bug was that fish did not match that behavior. I added explicit bash coverage in 1549407 so the #2333 callback shape is covered while keeping the fish regression test in place. The latest CI is green across the Go stable/oldstable matrix.