fix: skip all hidden flags in shell completion, not just bool flags

[vidigoat]

What type of PR is this?

• bug

What this PR does / why we need it:

printFlagSuggestions (used by the default shell-completion handler) is supposed to skip hidden flags, but it only skips hidden *BoolFlag values:

if bflag, ok := flag.(*BoolFlag); ok && bflag.Hidden {
    continue
}

The type assertion is to the concrete *BoolFlag type, so a hidden flag of any other type (*StringFlag, *IntFlag, *DurationFlag, ...) fails the assertion and is emitted into the completion output anyway. That exposes flags the author explicitly marked Hidden: true whenever a user hits <TAB>.

Repro (hidden *StringFlag):

cmd := &cli.Command{
    Name:                  "app",
    EnableShellCompletion: true,
    Commands: []*cli.Command{{
        Name: "sub",
        Flags: []cli.Flag{
            &cli.StringFlag{Name: "secret", Hidden: true},
            &cli.StringFlag{Name: "visible"},
        },
        Action: func(context.Context, *cli.Command) error { return nil },
    }},
}

app sub -<TAB> prints --secret alongside --visible, even though secret is hidden. A hidden *BoolFlag in the same list is correctly omitted.

Changes:

• help.go: replace the *BoolFlag-specific check with the existing VisibleFlag interface (IsVisible), which every flag type implements via FlagBase. This is the same mechanism visibleFlags and the flag-category code already use, so all hidden flags are now skipped regardless of concrete type.

Which issue(s) this PR fixes:

NONE

Testing

Added typical-flag-suggestion-hidden-non-bool to TestDefaultCompleteWithFlags, mirroring the existing typical-flag-suggestion-hidden-bool case but with a hidden *StringFlag. It fails on main (completion emits the hidden flag) and passes with this change. go test ./..., go vet ./..., and gofmt are clean.

Release Notes

Hidden flags of non-boolean types are no longer offered in shell completion (previously only hidden bool flags were skipped).

[dearchap]

Review

Verified locally: the new test typical-flag-suggestion-hidden-non-bool fails on main (emits --excellent) and passes with this change. Full test suite and go vet are clean.

The fix is correct and minimal. Using the VisibleFlag/IsVisible() interface is the same pattern already used by visibleFlags() and category.go, and every built-in flag type satisfies VisibleFlag via FlagBase, plus BoolWithInverseFlag and the external-flag wrapper, so no built-in type regresses. Custom flags implementing only Flag are unaffected.

Notes (non-blocking):

• This also fixes hidden *BoolWithInverseFlag, which was previously leaked too (the old *BoolFlag assertion didn't match that concrete type). Consistent with the intent, but there's no test covering it — consider adding a BoolWithInverseFlag case to the table.
• No exported API change, so no doc regeneration needed.

Nothing blocking. LGTM.