messages: Return shallow copy of message object.

When more than one outgoing webhook is configured,
the message which is send to the webhook bot passes
through finalize_payload function multiple times,
which mutated the message dict in a way that many keys
were lost from the dict obj.

This commit fixes that problem by having
`finalize_payload` return a shallow copy of the
incoming dict, instead of mutating it.  We still
mutate dicts inside of `post_process_dicts`, though,
for performance reasons.

This was slightly modified by @showell to fix the
`test_both_codepaths` test that was added concurrently
to this work.  (I used a slightly verbose style in the
tests to emphasize the transformation from `wide_dict`
to `narrow_dict`.)

I also removed a deepcopy call inside
`get_client_payload`, since we now no longer mutate
in `finalize_payload`.

Finally, I added some comments here and there.

For testing, I mostly protect against the root
cause of the bug happening again, by adding a line
to make sure that `sender_realm_id` does not get
wiped out from the "wide" dictionary.

A better test would exercise the actual code that
exposed the bug here by sending a message to a bot
with two or more services attached to it.  I will
do that in a future commit.

Fixes #14384

zerver/lib/outgoing_webhook.py

@@ -37,7 +37,7 @@ class GenericOutgoingWebhookService(OutgoingWebhookServiceInterface):
         info to the clients (so they don't have to compute
         it themselves).
         '''
-        MessageDict.finalize_payload(
+        message_dict = MessageDict.finalize_payload(
             event['message'],
             apply_markdown=False,
             client_gravatar=False,
@@ -45,7 +45,7 @@ class GenericOutgoingWebhookService(OutgoingWebhookServiceInterface):
         )
 
         request_data = {"data": event['command'],
-                        "message": event['message'],
+                        "message": message_dict,
                         "bot_email": self.user_profile.email,
                         "token": self.token,
                         "trigger": event['trigger']}