WIP thinking API support

- Allows specifying whether thinking mode should be on or not
- Templates get passed a new option so, e.g., qwen3's template can put
  `/think` or `/no_think` in the system prompt depending on the value of
  the setting
- Add parsing for thinking blocks in both streaming/non-streaming mode
- Update the CLI to make use of these changes

TODO:

- [ ] Don't parse thinking blocks when the user doesn't explicitly set
      the option, to maintain backwards compatibility
- [ ] Warning on CLI when using a non-thinking/older version of a model
      (with an old template)
- [ ] Wire up capabilities fully
- [x] Unify parsing for streaming/non-streaming
- [ ] Update templates
- [ ] Update python/js libraries
- [ ] How to handle differences in models wrt defaults and whether or
      not the thinking ability can even be controlled. If not specified
      by the user, should there be a default or should the template be
      able to check if it was explicitly set?

server/routes_generate_test.go

@@ -143,6 +143,24 @@ func TestGenerateChat(t *testing.T) {
 		}
 	})
 
+	t.Run("missing thinking capability", func(t *testing.T) {
+		w := createRequest(t, s.ChatHandler, api.ChatRequest{
+			Model: "test",
+			Messages: []api.Message{
+				{Role: "user", Content: "Hello!"},
+			},
+			Thinking: true,
+		})
+
+		if w.Code != http.StatusBadRequest {
+			t.Errorf("expected status 400, got %d", w.Code)
+		}
+
+		if diff := cmp.Diff(w.Body.String(), `{"error":"registry.ollama.ai/library/test:latest does not support thinking"}`); diff != "" {
+			t.Errorf("mismatch (-got +want):\n%s", diff)
+		}
+	})
+
 	t.Run("missing model", func(t *testing.T) {
 		w := createRequest(t, s.ChatHandler, api.ChatRequest{})
 		if w.Code != http.StatusBadRequest {