* feat: include connection id in the plugin context
* refactor: group ConnectionId next to SessionId in plugin Context
Addresses review feedback to keep related identifier fields adjacent.
* fix(files): forward Connection-Id on file uploads to plugin hooks
The webapp uploadFile XHR didn't attach the Connection-Id header, so
FileWillBeUploaded plugin hooks always received an empty ConnectionId.
Read it from the websocket selector and set it on the request, matching
how drafts and channel bookmarks already do it. Adds a server-side test
asserting the connection id propagates through pluginContext.
* fix(lint): reorder file_actions imports to satisfy import/order
* Document ConnectionId on request.Context