fix: When a Grammar combines flags with passthrough args, see if an u…

…nrecognized flag may be treated as a positional argument

### Issue
Given a grammar like this:
```golang
var cli struct {
	Args []string `arg:"" optional:"" passthrough:""`
}
```

The first positional argument implies that it was preceded by `--`, so subsequent flags are not parsed.

If Kong parses `cli 1 --unknown 3`, it will populate `Args` with `[]string{"1", "--unknown", "3"}`.
However, if Kong parses `cli --unknown 2 3`, it will fail saying that `--unknown` is an unrecognized flag.

### Proposal
This commit changes the parser so that if an unknown flag _could_ be treated as the first passthrough argument, it is.

After this change, if Kong parses `cli --unknown 2 3`, it will populate `Args` with `[]string{"--unknown", "2", "3"}`.

context.go

@@ -420,12 +420,22 @@ func (c *Context) trace(node *Node) (err error) { //nolint: gocyclo
 
 		case FlagToken:
 			if err := c.parseFlag(flags, token.String()); err != nil {
-				return err
+				if isUnknownFlagError(err) && positional < len(node.Positional) && node.Positional[positional].Passthrough {
+					c.scan.Pop()
+					c.scan.PushTyped(token.String(), PositionalArgumentToken)
+				} else {
+					return err
+				}
 			}
 
 		case ShortFlagToken:
 			if err := c.parseFlag(flags, token.String()); err != nil {
-				return err
+				if isUnknownFlagError(err) && positional < len(node.Positional) && node.Positional[positional].Passthrough {
+					c.scan.Pop()
+					c.scan.PushTyped(token.String(), PositionalArgumentToken)
+				} else {
+					return err
+				}
 			}
 
 		case FlagValueToken:
@@ -728,9 +738,19 @@ func (c *Context) parseFlag(flags []*Flag, match string) (err error) {
 		c.Path = append(c.Path, &Path{Flag: flag})
 		return nil
 	}
-	return findPotentialCandidates(match, candidates, "unknown flag %s", match)
+	return &UnknownFlagError{Cause: findPotentialCandidates(match, candidates, "unknown flag %s", match)}
+}
+
+func isUnknownFlagError(err error) bool {
+	var unknown *UnknownFlagError
+	return errors.As(err, &unknown)
 }
 
+type UnknownFlagError struct{ Cause error }
+
+func (e *UnknownFlagError) Unwrap() error { return e.Cause }
+func (e *UnknownFlagError) Error() string { return e.Cause.Error() }
+
 // Call an arbitrary function filling arguments with bound values.
 func (c *Context) Call(fn any, binds ...interface{}) (out []interface{}, err error) {
 	fv := reflect.ValueOf(fn)

kong_test.go

@@ -1551,6 +1551,12 @@ func TestPassthroughArgs(t *testing.T) {
 			"",
 			[]string{"--flag", "foobar"},
 		},
+		{
+			"UnrecognizedFlagAndArgs",
+			[]string{"--unrecognized-flag", "something"},
+			"",
+			[]string{"--unrecognized-flag", "something"},
+		},
 	}
 	for _, test := range tests {
 		test := test