Previously, if the first user message was sent with the command
invocation, e.g.:
```
$ cargo run --bin codex 'hello'
```
Then the user message was added as the first entry in the history and
then `is_first_event` would be `false` here:
031df77dfb/codex-rs/tui/src/conversation_history_widget.rs (L178-L179)
which would prevent the "welcome" message with things like the the model
version from displaying.
The fix in this PR is twofold:
* Reorganize the logic so the `ChatWidget` constructor stores
`initial_user_message` rather than sending it right away. Now inside
`handle_codex_event()`, it waits for the `SessionConfigured` event and
sends the `initial_user_message`, if it exists.
* In `conversation_history_widget.rs`, `add_session_info()` checks to
see whether a `WelcomeMessage` exists in the history when determining
the value of `has_welcome_message`. By construction, we expect that
`WelcomeMessage` is always the first message (in which case the existing
`let is_first_event = self.entries.is_empty();` logic would be sound),
but we decide to be extra defensive in case an `EventMsg::Error` is
processed before `EventMsg::SessionConfigured`.
f8b6b1db81