api-changelog: Add entry for feature level 193 updates.

Adds an entry to the API changelog for feature level 193. The
changes documented here are from commit 3802503a8a.

api_docs/changelog.md

@@ -20,6 +20,16 @@ format used by the Zulip server that they are interacting with.
 
 ## Changes in Zulip 8.0
 
+**Feature level 193**
+
+* [`POST /messages/{message_id}/reactions`](/api/add-reaction),
+  [`DELETE /messages/{message_id}/reactions`](/api/remove-reaction):
+  Endpoints return specific error responses if an emoji reaction
+  already exists when adding a reaction (`"code": "REACTION_ALREADY_EXISTS"`)
+  or if an emoji reaction does not exist when deleting a reaction
+  (`"code": "REACTION_DOES_NOT_EXIST"`). Previously, these errors
+  returned the `"BAD_REQUEST"` code.
+
 **Feature level 192**
 
 * [`GET /events`](/api/get-events): Stream creation events are now

zerver/openapi/zulip.yaml

@@ -6680,7 +6680,8 @@ paths:
           content:
             application/json:
               schema:
-                allOf:
+                oneOf:
+                  - allOf:
                       - $ref: "#/components/schemas/CodedError"
                       - example:
                           {
@@ -6690,7 +6691,19 @@ paths:
                           }
                         description: |
                           An example JSON error response for when the emoji code is invalid:
+                  - allOf:
+                      - $ref: "#/components/schemas/CodedError"
+                      - example:
+                          {
+                            "result": "error",
+                            "msg": "Reaction already exists.",
+                            "code": "REACTION_ALREADY_EXISTS",
+                          }
+                        description: |
+                          An example JSON error response for when the reaction already exists.
 
+                          **Changes**: New in Zulip 8.0 (feature level 193). Previously, this
+                          error returned the `"BAD_REQUEST"` code.
     delete:
       operationId: remove-reaction
       summary: Remove an emoji reaction
@@ -6729,16 +6742,30 @@ paths:
           content:
             application/json:
               schema:
-                allOf:
+                oneOf:
+                  - allOf:
                       - $ref: "#/components/schemas/CodedError"
                       - example:
                           {
                             "result": "error",
-                        "msg": "Invalid message(s)",
+                            "msg": "Invalid emoji code",
                             "code": "BAD_REQUEST",
                           }
                         description: |
                           An example JSON error response for when the emoji code is invalid:
+                  - allOf:
+                      - $ref: "#/components/schemas/CodedError"
+                      - example:
+                          {
+                            "result": "error",
+                            "msg": "Reaction doesn't exist.",
+                            "code": "REACTION_DOES_NOT_EXIST",
+                          }
+                        description: |
+                          An example JSON error response for when the reaction does not exist.
+
+                          **Changes**: New in Zulip 8.0 (feature level 193). Previously, this
+                          error returned the `"BAD_REQUEST"` code.
 
   /messages/{message_id}/read_receipts:
     get: