[MM-54730] Don't use old hardcoded rule for validating imported posts (#25823)

* don't use old hardcoded rule for validating imported posts

* fix http verb in doc

* Use client config

* Handle local mode

* E2E tests

* Enforce default if unable to use real limit

* Unit tests

* Fix tests

* Use model.PostMessageMaxRunesV2 as lower default

* Update direct post message length validation

---------

Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: streamer45 <cstcld91@gmail.com>

server/channels/api4/config_local.go

@@ -21,6 +21,7 @@ func (api *API) InitConfigLocal() {
 	api.BaseRoutes.APIRoot.Handle("/config/patch", api.APILocal(localPatchConfig)).Methods("PUT")
 	api.BaseRoutes.APIRoot.Handle("/config/reload", api.APILocal(configReload)).Methods("POST")
 	api.BaseRoutes.APIRoot.Handle("/config/migrate", api.APILocal(localMigrateConfig)).Methods("POST")
+	api.BaseRoutes.APIRoot.Handle("/config/client", api.APILocal(localGetClientConfig)).Methods("GET")
 }
 
 func localGetConfig(c *Context, w http.ResponseWriter, r *http.Request) {
@@ -173,3 +174,24 @@ func localMigrateConfig(c *Context, w http.ResponseWriter, r *http.Request) {
 	auditRec.Success()
 	ReturnStatusOK(w)
 }
+
+func localGetClientConfig(c *Context, w http.ResponseWriter, r *http.Request) {
+	auditRec := c.MakeAuditRecord("localGetClientConfig", audit.Fail)
+	defer c.LogAuditRec(auditRec)
+
+	format := r.URL.Query().Get("format")
+
+	if format == "" {
+		c.Err = model.NewAppError("getClientConfig", "api.config.client.old_format.app_error", nil, "", http.StatusNotImplemented)
+		return
+	}
+
+	if format != "old" {
+		c.SetInvalidParam("format")
+		return
+	}
+
+	auditRec.Success()
+
+	w.Write([]byte(model.MapToJSON(c.App.Srv().Platform().ClientConfigWithComputed())))
+}