add suffix support to generate endpoint

this change is triggered by the presence of "suffix", particularly
useful for code completion tasks

template/template_test.go

@@ -359,3 +359,38 @@ Answer: `,
 		})
 	}
 }
+
+func TestExecuteWithSuffix(t *testing.T) {
+	tmpl, err := Parse(`{{- if .Suffix }}<PRE> {{ .Prompt }} <SUF>{{ .Suffix }} <MID>
+{{- else }}{{ .Prompt }}
+{{- end }}`)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cases := []struct {
+		name   string
+		values Values
+		expect string
+	}{
+		{
+			"message", Values{Messages: []api.Message{{Role: "user", Content: "hello"}}}, "hello",
+		},
+		{
+			"prompt suffix", Values{Prompt: "def add(", Suffix: "return x"}, "<PRE> def add( <SUF>return x <MID>",
+		},
+	}
+
+	for _, tt := range cases {
+		t.Run(tt.name, func(t *testing.T) {
+			var b bytes.Buffer
+			if err := tmpl.Execute(&b, tt.values); err != nil {
+				t.Fatal(err)
+			}
+
+			if diff := cmp.Diff(b.String(), tt.expect); diff != "" {
+				t.Errorf("mismatch (-got +want):\n%s", diff)
+			}
+		})
+	}
+}