fix: check if buffer is in chat window to validate chat being visible (#330)

* fix: check if buffer is in chat window to validate chat being visible

This prevents someone replacing buffer in copilot chat window with other
buffer and chat not being able to recover

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: deathbeam

lua/CopilotChat/chat.lua

@@ -61,7 +61,9 @@ local Chat = class(function(self, mark_ns, help, on_buf_create)
 end, Overlay)
 
 function Chat:visible()
-  return self.winnr and vim.api.nvim_win_is_valid(self.winnr)
+  return self.winnr
+    and vim.api.nvim_win_is_valid(self.winnr)
+    and vim.api.nvim_win_get_buf(self.winnr) == self.bufnr
 end
 
 function Chat:active()