web action extensions are not known to a web action

`.json`, `.html`, `.svg` are not known to a web action wrt the accept content type and should be.

in general these extensions are intended as syntactic sugar for `.http` for using off the shelf actions which automatic creation of the `.http` response with automatic `headers = {}` and `statusCode = 200` (the headers are empty because of a bug see #2443 but should include the standard defaults).

if the action is already prepared to handle `.http` requests but the action is called with the `.json` extension, it is not aware of the intended accept-type because the controller does not pass this information to the action.

Start by tracing the controller path for .json, .html, and .svg web action requests and how it invokes actions prepared for .http requests. Verify how the extension should convey the intended accept content type, then add coverage showing that the action receives it while preserving the automatic response defaults.

Written by the indexing model from the issue text.