Remove Assist command execution and chat feature (#40806)

* Remove Assist command execution and chat feature

The changes remove tests and related code for command execution in the assist package and web command handling.

* Remove Teleport Assist chat from CHANGELOG.md

The changes in this commit reflect the removal of the Teleport Assist chat from the CHANGELOG documentation of Teleport version 16. The Assist feature continues to be available in the SSH Web Terminal and Audit Monitoring.

* Remove assist chat from the test plan

.github/ISSUE_TEMPLATE/testplan.md

@@ -1504,17 +1504,6 @@ Assist test plan is in the core section instead of WebUI as most functionality i
   - [ ] Assist is enabled by default in the Cloud Team plan.
   - [ ] Assist is always disabled when etcd is used as a backend.
 
-- Conversations
-  - [ ] A new conversation can be started.
-  - [ ] SSH command can be executed on one server.
-  - [ ] SSH command can be executed on multiple servers.
-  - [ ] SSH command can be executed on a node with per session MFA enabled.
-  - [ ] Execution output is explained when it fits the context window.
-  - [ ] Assist can list all nodes/execute a command on all nodes (using embeddings).
-  - [ ] Access request can be created.
-  - [ ] Access request is created when approved.
-  - [ ] Conversation title is set after the first message.
-
 - SSH integration
   - [ ] Assist icon is visible in WebUI's Terminal
   - [ ] A Bash command can be generated in the above window.

CHANGELOG.md

@@ -12,6 +12,11 @@ Opsgenie plugin users, role annotations must now contain
 See [the Opsgenie plugin documentation](docs/pages/access-controls/access-request-plugins/opsgenie.mdx)
 for setup instructions.
 
+#### Teleport Assist chat has been remove.
+
+Teleport Assist chat has been removed from Teleport 16. Assist is still available
+in the SSH Web Terminal and Audit Monitoring.
+
 ## 15.0.0 (xx/xx/24)
 
 ### New features