feat: add an endpoint to manually pause a coder task (#21889)

Closes https://github.com/coder/internal/issues/1261.

This pull request adds an endpoint to pause coder tasks by stopping the
underlying workspace.
* Instead of `POST /api/v2/tasks/{user}/{task}/pause`, the endpoint is
currently experimental.
* We do not currently set the build reason to `task_manual_pause`,
because build reasons are currently only used on stop transitions.

coderd/aitasks.go

@@ -1244,3 +1244,63 @@ func (api *API) postWorkspaceAgentTaskLogSnapshot(rw http.ResponseWriter, r *htt
 
 	rw.WriteHeader(http.StatusNoContent)
 }
+
+// @Summary Pause task
+// @ID pause-task
+// @Security CoderSessionToken
+// @Accept json
+// @Tags Tasks
+// @Param user path string true "Username, user ID, or 'me' for the authenticated user"
+// @Param task path string true "Task ID" format(uuid)
+// @Success 202 {object} codersdk.PauseTaskResponse
+// @Router /tasks/{user}/{task}/pause [post]
+func (api *API) pauseTask(rw http.ResponseWriter, r *http.Request) {
+	var (
+		ctx    = r.Context()
+		apiKey = httpmw.APIKey(r)
+		task   = httpmw.TaskParam(r)
+	)
+
+	if !task.WorkspaceID.Valid {
+		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+			Message: "Task does not have a workspace.",
+		})
+		return
+	}
+
+	workspace, err := api.Database.GetWorkspaceByID(ctx, task.WorkspaceID.UUID)
+	if err != nil {
+		if httpapi.Is404Error(err) {
+			httpapi.ResourceNotFound(rw)
+			return
+		}
+		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+			Message: "Internal error fetching task workspace.",
+			Detail:  err.Error(),
+		})
+		return
+	}
+
+	buildReq := codersdk.CreateWorkspaceBuildRequest{
+		Transition: codersdk.WorkspaceTransitionStop,
+		Reason:     codersdk.CreateWorkspaceBuildReasonTaskManualPause,
+	}
+	build, err := api.postWorkspaceBuildsInternal(
+		ctx,
+		apiKey,
+		workspace,
+		buildReq,
+		func(action policy.Action, object rbac.Objecter) bool {
+			return api.Authorize(r, action, object)
+		},
+		audit.WorkspaceBuildBaggageFromRequest(r),
+	)
+	if err != nil {
+		httperror.WriteWorkspaceBuildError(ctx, rw, err)
+		return
+	}
+
+	httpapi.Write(ctx, rw, http.StatusAccepted, codersdk.PauseTaskResponse{
+		WorkspaceBuild: &build,
+	})
+}