Feature/oc 1316 add support for system message to single user (#1351)

* add public func to lookup a ChatClient by its clientId

* add facility to send a system message directly to a user

* add clientId field to UserEvent

* implement simple http endpoint to send a message to a user

* let mux handle new directSystemMessageToUser endpoint

* add ClientId to UserEvents across the codebase

* render body of system-message to client

* add clientId to Chat-Message

* add tests showing how url-parsing should work

* add simple rest endpoint helpers for parameter-parsing and easy routing

* use newly added rest-endpoint helper to rout to Client-Messaging controller

* use safe "ReadRestUrlParameter" to parse ClientId

* remove empty HandleFunc in router

* set Header directly to prevent built-in (platform-dependent) canonicalization to kick in

* fix typo in "Parameter" message

* remove debug-logging of HTTP headers in REST-helpers

* convert to uint32 to prevent overruns when converting to wraptype uint later on

* resolve linter-ouchies

* resolve linter potential nil-deref warning

* document the SendSystemMessageToClient endpoint in swaggerdoc

* remove clientId assignment causing potential nil dereference in userDisabledEvent-case

as the clientId isn't relevant here anyway

* make findClientById private, so its not accessible outside of core/chat

* remove redundant string type hint

* Update PR based on linter requirements

Co-authored-by: Raffael Rehberger <raffael@rtrace.io>
Co-authored-by: Gabe Kangas <gabek@real-ity.com>

openapi.yaml

@@ -1398,6 +1398,58 @@ paths:
                     type: string
                     example: sent
 
+  /api/integrations/chat/system/client/{clientId}:
+    post:
+      summary: Send system chat message to a client, identified by its ClientId
+      description: Send a chat message on behalf of the system/server to a single client.
+      tags: ["Integrations"]
+      security:
+        - AccessToken: []
+      parameters:
+        - name: clientId
+          in: path
+          description: Client ID (a unique numeric Id, identifying the client connection)
+          required: true
+          schema:
+            type: integer
+            format: int64
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              type: object
+              required:
+                - "body"
+              properties:
+                body:
+                  type: string
+                  description: The message text that will be sent to the client.
+                  example: "What a beautiful day. I love it"
+      responses:
+        "200":
+          description: Message was sent.
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  success:
+                    type: boolean
+                    example: true
+                  messages:
+                    type: string
+                    example: sent
+        "500":
+          description: Message was not sent to the client
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  error: 
+                    type: string
+                    description: message explaining what went wrong sending the message to the client
 
   /api/admin/accesstokens/create:
     post: