fix(web): issue2551356. Add etag header for not-modified (304) request. When a 304 is returned to a conditional request for a static file, print an ETag for the response. ETag was always sent with a 200 response. This also adds initial support for if-none-match conditional requests for static files. Changes: Refactors the if-modified-since code out to a method. It moves a file stat call from serve_static_file to _serve_file so that an etag can be generated by both serve_static_file and serve_file which call _serve_file. Tests added. This does not test the codepath where serve_file pulls content from the database rather than from a local file on disk. Test mocking _serve_file changed to account for 5th argument to serve_file BREAKING CHANGE: function signature for client.py-Client::_serve_file() now has 5 not 4 parameters (added etag param). Since this is a "hidden" method I am not too worried about it.

--- a/CHANGES.txt	Sun Dec 08 21:36:29 2024 -0500
+++ b/CHANGES.txt	Tue Dec 10 16:06:13 2024 -0500
@@ -36,6 +36,9 @@
 - issue2551289 - Invalid REST Accept header with post/put performs
   change before returning 406. Error before making any changes to the
   db if we can't respond with requested format. (John Rouillard)
+- issue2551356 - Add etag header when If-Modified-Since GET request
+  returns not-modified (304). Breaking change to function signature
+  for client.py-Client::_serve_file(). (John Rouillard)
 
 Features: