feat(chat): Add delete chat session endpoint and UI (#12112)

## Summary

Adds the ability to delete chat sessions from the CoPilot interface.

## Changes

### Backend
- Add `DELETE /api/chat/sessions/{session_id}` endpoint in `routes.py`
- Returns 204 on success, 404 if not found or not owned by user
- Reuses existing `delete_chat_session` function from `model.py`

### Frontend
- Add delete button (trash icon) that appears on hover for each chat
session
- Add confirmation dialog before deletion using existing
`DeleteConfirmDialog` component
- Refresh session list after successful delete
- Clear current session selection if the deleted session was active
- Update OpenAPI spec with new endpoint

## Testing

1. Hover over a chat session in sidebar → trash icon appears
2. Click trash icon → confirmation dialog
3. Confirm deletion → session removed, list refreshes
4. If deleted session was active, selection is cleared

## Screenshots

Delete button appears on hover, confirmation dialog on click.

## Related Issues

Closes SECRT-1928

<!-- greptile_comment -->

<h2>Greptile Overview</h2>

<details><summary><h3>Greptile Summary</h3></summary>

Adds the ability to delete chat sessions from the CoPilot interface — a
new `DELETE /api/chat/sessions/{session_id}` backend endpoint and a
corresponding delete button with confirmation dialog in the
`ChatSidebar` frontend component.

- **Backend route** (`routes.py`): Clean implementation reusing the
existing `delete_chat_session` model function with proper auth guards
and 204/404 responses. No issues.
- **Frontend** (`ChatSidebar.tsx`): Adds hover-visible trash icon per
session, confirmation dialog, mutation with cache invalidation, and
active session clearing on delete. However, it uses a `__legacy__`
component (`DeleteConfirmDialog`) which violates the project's style
guide — new code should use the modern design system components. Error
handling only logs to console without user-facing feedback (project
convention is to use toast notifications for mutation errors).
`isDeleting` is destructured but unused.
- **OpenAPI spec** updated correctly.
- **Unrelated file included**:
`notes/plan-SECRT-1959-graph-edge-desync.md` is a planning document for
a different ticket and should be removed from this PR. The `notes/`
directory is newly introduced and both plan files should be reconsidered
for inclusion.
</details>


<details><summary><h3>Confidence Score: 3/5</h3></summary>

- Functionally correct but has style guide violations and includes
unrelated files that should be addressed before merge.
- The core feature implementation (backend DELETE endpoint and frontend
mutation logic) is sound and follows existing patterns. Score is lowered
because: (1) the frontend uses a legacy component explicitly prohibited
by the project's style guide, (2) mutation errors are not surfaced to
the user, and (3) the PR includes an unrelated planning document for a
different ticket.
- Pay close attention to `ChatSidebar.tsx` for the legacy component
import and error handling, and
`notes/plan-SECRT-1959-graph-edge-desync.md` which should be removed.
</details>


<details><summary><h3>Sequence Diagram</h3></summary>

```mermaid
sequenceDiagram
    participant User
    participant ChatSidebar as ChatSidebar (Frontend)
    participant ReactQuery as React Query
    participant API as DELETE /api/chat/sessions/{id}
    participant Model as model.delete_chat_session
    participant DB as db.delete_chat_session (Prisma)
    participant Redis as Redis Cache

    User->>ChatSidebar: Click trash icon on session
    ChatSidebar->>ChatSidebar: Show DeleteConfirmDialog
    User->>ChatSidebar: Confirm deletion
    ChatSidebar->>ReactQuery: deleteSession({ sessionId })
    ReactQuery->>API: DELETE /api/chat/sessions/{session_id}
    API->>Model: delete_chat_session(session_id, user_id)
    Model->>DB: delete_many(where: {id, userId})
    DB-->>Model: bool (deleted count > 0)
    Model->>Redis: Delete session cache key
    Model->>Model: Clean up session lock
    Model-->>API: True
    API-->>ReactQuery: 204 No Content
    ReactQuery->>ChatSidebar: onSuccess callback
    ChatSidebar->>ReactQuery: invalidateQueries(sessions list)
    ChatSidebar->>ChatSidebar: Clear sessionId if deleted was active
```
</details>


<sub>Last reviewed commit: 44a92c6</sub>

<!-- greptile_other_comments_section -->

<details><summary><h4>Context used (3)</h4></summary>

- Context from `dashboard` - autogpt_platform/frontend/CLAUDE.md
([source](https://app.greptile.com/review/custom-context?memory=39861924-d320-41ba-a1a7-a8bff44f780a))
- Context from `dashboard` - autogpt_platform/frontend/CONTRIBUTING.md
([source](https://app.greptile.com/review/custom-context?memory=cc4f1b17-cb5c-4b63-b218-c772b48e20ee))
- Context from `dashboard` - autogpt_platform/CLAUDE.md
([source](https://app.greptile.com/review/custom-context?memory=6e9dc5dc-8942-47df-8677-e60062ec8c3a))
</details>


<!-- /greptile_comment -->

---------

Co-authored-by: Zamil Majdy <zamil.majdy@agpt.co>

autogpt_platform/frontend/src/app/api/openapi.json

@@ -1151,6 +1151,36 @@
       }
     },
     "/api/chat/sessions/{session_id}": {
+      "delete": {
+        "tags": ["v2", "chat", "chat"],
+        "summary": "Delete Session",
+        "description": "Delete a chat session.\n\nPermanently removes a chat session and all its messages.\nOnly the owner can delete their sessions.\n\nArgs:\n    session_id: The session ID to delete.\n    user_id: The authenticated user's ID.\n\nReturns:\n    204 No Content on success.\n\nRaises:\n    HTTPException: 404 if session not found or not owned by user.",
+        "operationId": "deleteV2DeleteSession",
+        "security": [{ "HTTPBearerJWT": [] }],
+        "parameters": [
+          {
+            "name": "session_id",
+            "in": "path",
+            "required": true,
+            "schema": { "type": "string", "title": "Session Id" }
+          }
+        ],
+        "responses": {
+          "204": { "description": "Successful Response" },
+          "401": {
+            "$ref": "#/components/responses/HTTP401NotAuthenticatedError"
+          },
+          "404": { "description": "Session not found or access denied" },
+          "422": {
+            "description": "Validation Error",
+            "content": {
+              "application/json": {
+                "schema": { "$ref": "#/components/schemas/HTTPValidationError" }
+              }
+            }
+          }
+        }
+      },
       "get": {
         "tags": ["v2", "chat", "chat"],
         "summary": "Get Session",