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/error.rs

@@ -55,7 +55,7 @@ pub enum CodexErr {
 
     /// Returned by run_command_stream when the user pressed Ctrl‑C (SIGINT). Session uses this to
     /// surface a polite FunctionCallOutput back to the model instead of crashing the CLI.
-    #[error("interrupted (Ctrl‑C)")]
+    #[error("interrupted (Ctrl-C)")]
     Interrupted,
 
     /// Unexpected HTTP status code.
@@ -97,8 +97,28 @@ pub enum CodexErr {
     #[error(transparent)]
     TokioJoin(#[from] JoinError),
 
-    #[error("missing environment variable {0}")]
-    EnvVar(&'static str),
+    #[error("{0}")]
+    EnvVar(EnvVarError),
+}
+
+#[derive(Debug)]
+pub struct EnvVarError {
+    /// Name of the environment variable that is missing.
+    pub var: String,
+
+    /// Optional instructions to help the user get a valid value for the
+    /// variable and set it.
+    pub instructions: Option<String>,
+}
+
+impl std::fmt::Display for EnvVarError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "Missing environment variable: `{}`.", self.var)?;
+        if let Some(instructions) = &self.instructions {
+            write!(f, " {instructions}")?;
+        }
+        Ok(())
+    }
 }
 
 impl CodexErr {