feat: support the chat completions API in the Rust CLI (#862)

This is a substantial PR to add support for the chat completions API,
which in turn makes it possible to use non-OpenAI model providers (just
like in the TypeScript CLI):

* It moves a number of structs from `client.rs` to `client_common.rs` so
they can be shared.
* It introduces support for the chat completions API in
`chat_completions.rs`.
* It updates `ModelProviderInfo` so that `env_key` is `Option<String>`
instead of `String` (for e.g., ollama) and adds a `wire_api` field
* It updates `client.rs` to choose between `stream_responses()` and
`stream_chat_completions()` based on the `wire_api` for the
`ModelProviderInfo`
* It updates the `exec` and TUI CLIs to no longer fail if the
`OPENAI_API_KEY` environment variable is not set
* It updates the TUI so that `EventMsg::Error` is displayed more
prominently when it occurs, particularly now that it is important to
alert users to the `CodexErr::EnvVar` variant.
* `CodexErr::EnvVar` was updated to include an optional `instructions`
field so we can preserve the behavior where we direct users to
https://platform.openai.com if `OPENAI_API_KEY` is not set.
* Cleaned up the "welcome message" in the TUI to ensure the model
provider is displayed.
* Updated the docs in `codex-rs/README.md`.

To exercise the chat completions API from OpenAI models, I added the
following to my `config.toml`:

```toml
model = "gpt-4o"
model_provider = "openai-chat-completions"

[model_providers.openai-chat-completions]
name = "OpenAI using Chat Completions"
base_url = "https://api.openai.com/v1"
env_key = "OPENAI_API_KEY"
wire_api = "chat"
```

Though to test a non-OpenAI provider, I installed ollama with mistral
locally on my Mac because ChatGPT said that would be a good match for my
hardware:

```shell
brew install ollama
ollama serve
ollama pull mistral
```

Then I added the following to my `~/.codex/config.toml`:

```toml
model = "mistral"
model_provider = "ollama"
```

Note this code could certainly use more test coverage, but I want to get
this in so folks can start playing with it.

For reference, I believe https://github.com/openai/codex/pull/247 was
roughly the comparable PR on the TypeScript side.

codex-rs/core/src/conversation_history.rs

@@ -0,0 +1,48 @@
+use crate::models::ResponseItem;
+
+/// Transcript of conversation history that is needed:
+/// - for ZDR clients for which previous_response_id is not available, so we
+///   must include the transcript with every API call. This must include each
+///   `function_call` and its corresponding `function_call_output`.
+/// - for clients using the "chat completions" API as opposed to the
+///   "responses" API.
+#[derive(Debug, Clone)]
+pub(crate) struct ConversationHistory {
+    /// The oldest items are at the beginning of the vector.
+    items: Vec<ResponseItem>,
+}
+
+impl ConversationHistory {
+    pub(crate) fn new() -> Self {
+        Self { items: Vec::new() }
+    }
+
+    /// Returns a clone of the contents in the transcript.
+    pub(crate) fn contents(&self) -> Vec<ResponseItem> {
+        self.items.clone()
+    }
+
+    /// `items` is ordered from oldest to newest.
+    pub(crate) fn record_items<I>(&mut self, items: I)
+    where
+        I: IntoIterator<Item = ResponseItem>,
+    {
+        for item in items {
+            if is_api_message(&item) {
+                // Note agent-loop.ts also does filtering on some of the fields.
+                self.items.push(item.clone());
+            }
+        }
+    }
+}
+
+/// Anything that is not a system message or "reasoning" message is considered
+/// an API message.
+fn is_api_message(message: &ResponseItem) -> bool {
+    match message {
+        ResponseItem::Message { role, .. } => role.as_str() != "system",
+        ResponseItem::FunctionCall { .. } => true,
+        ResponseItem::FunctionCallOutput { .. } => true,
+        _ => false,
+    }
+}