fix: sanitize loaded chat history

Ensure that chat history loaded from a file has resemblance of correct
data. Namely title and timestamp are present and are strings, and
entires, messages, and todos are lists. In case of inconsistencies
replace with empty/default data.

This should help with #2584.

More changes are needed to sanitize individual entries.

lua/avante/types.lua

@@ -503,9 +503,9 @@ vim.g.avante_login = vim.g.avante_login
 ---@class avante.ChatHistory
 ---@field title string
 ---@field timestamp string
----@field messages avante.HistoryMessage[] | nil
----@field entries avante.ChatHistoryEntry[] | nil
----@field todos avante.TODO[] | nil
+---@field messages avante.HistoryMessage[]
+---@field entries avante.ChatHistoryEntry[]
+---@field todos avante.TODO[]
 ---@field memory avante.ChatMemory | nil
 ---@field filename string
 ---@field system_prompt string | nil