When a user provides a custom `auto_approve` list in their TOML
config (e.g. to add an MCP tool), serde replaces the built-in
defaults instead of merging. This causes default safe tools like
`weather`, `calculator`, and `file_read` to lose auto-approve
status and get silently denied in non-interactive channel runs.
Add `ensure_default_auto_approve()` which merges built-in entries
into the user's list after deserialization, preserving user
additions while guaranteeing defaults are always present. Users
who want to require approval for a default tool can use
`always_ask`, which takes precedence.
Closes#4247
9731367ece