routes: fix built-in renderers for api/generate

Made it so when api/generate builds up a message array and generates the
prompt it now goes through the same function as `api/chat` for
consistency. This is where we hook the optional built-in renderers to
bypass templates, which was missing for `api/generate` before this
change.

Closes: #12578

server/routes_debug_test.go

@@ -146,7 +146,7 @@ func TestGenerateDebugRenderOnly(t *testing.T) {
 				DebugRenderOnly: true,
 			},
 			expectDebug:     true,
-			expectTemplate:  "[img-0]\n\nDescribe this image",
+			expectTemplate:  "[img-0]Describe this image",
 			expectNumImages: 1,
 		},
 		{