assistant: Do not create new context on load (#29480)

Closes https://github.com/zed-industries/zed/issues/27673
Closes https://github.com/zed-industries/zed/issues/29344
Closes #29863 

This PR fixes an issue where Zed was showing no language and `4:1` as a
line/column value on startup, as described in the linked issues. You can
actually see in the first issue that the user also experiences the same
issue as described in the second one, as his line/column value is
noticably also `4:1`.


https://github.com/user-attachments/assets/bb60e387-f4b8-4e05-80b3-4dadf1a01262

This issue arises because on assistant panel load, a new context is
created and its editor focused. However, the editor is not visible
despite having focus. The content for the editor for a new context is
`\n\n\n` and the cursor is inserted directly after that - this is where
the line:column position `4:1` comes from. For the assistant panel
editor, the language is intentionally hidden, this is why the language
is not shown on workspace load.
The issue is only present for as long as the user does not focus and
edit another editor, then that instance is focused and everything starts
to work properly again.

As this issue only arises with the old assistant panel, some staff
members were unable to reproduce in the linked issues. Once you set
`export ZED_DISABLE_STAFF=1` in your environment, you should also be
able to reproduce this issue consistently.

--- 

This PR fixes the issue by not creating a new context on assistant panel
load. This should not cause any regressions; every other code path I
checked creates a new context if no context is yet present.
Additionally, this also seems somewhat more reasonable, as users which
have the assistant panel disabled will never need a new context anyway,
so no context should be created.

In the following video, you can see this fixes the issue when the
assistant panel was not open the last time Zed was opened. If the panel
was open before Zed was closed, we will still properly focus the panel
and then the `4:1` will show again, which in that case is correct. The
assistant panel editor is focused and the missing language as well as
the line number then match what the user sees, experiences and expects.


https://github.com/user-attachments/assets/224a786b-52c7-4212-bccb-dff6d9db62c3


Release Notes:

- Fixed an issue where Zed would show no language and an incorrect
line/column value on startup.

Co-authored-by: Peter Tripp <peter@zed.dev>

crates/assistant/src/assistant_panel.rs

@@ -320,7 +320,7 @@ impl AssistantPanel {
 
         let watch_client_status = Self::watch_client_status(workspace.client().clone(), window, cx);
 
-        let mut this = Self {
+        Self {
             pane,
             workspace: workspace.weak_handle(),
             width: None,
@@ -336,9 +336,7 @@ impl AssistantPanel {
             client_status: None,
             watch_client_status: Some(watch_client_status),
             show_zed_ai_notice: false,
-        };
-        this.new_context(window, cx);
-        this
+        }
     }
 
     pub fn toggle_focus(