fix: recognize spot-only tokens in IsToken

Gate token detection via spot symbol resolution so chat flow accepts spot-only tokens, and add regression coverage for the fallback path.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/helper/binancex/resolver_test.go

@@ -137,6 +137,14 @@ func TestIsToken(t *testing.T) {
 			},
 			want: true,
 		},
+		{
+			name:  "spot only token fallback",
+			input: "abc",
+			marketStub: &resolverMarketStub{
+				spotPairs: map[string]bool{"ABCUSDT": true},
+			},
+			want: true,
+		},
 		{
 			name:       "non alphanumeric input",
 			input:      "bad!",