feat: add support for login with ChatGPT (#1212)

This does not implement the full Login with ChatGPT experience, but it
should unblock people.

**What works**

* The `codex` multitool now has a `login` subcommand, so you can run
`codex login`, which should write `CODEX_HOME/auth.json` if you complete
the flow successfully. The TUI will now read the `OPENAI_API_KEY` from
`auth.json`.
* The TUI should refresh the token if it has expired and the necessary
information is in `auth.json`.
* There is a `LoginScreen` in the TUI that tells you to run `codex
login` if both (1) your model provider expects to use `OPENAI_API_KEY`
as its env var, and (2) `OPENAI_API_KEY` is not set.

**What does not work**

* The `LoginScreen` does not support the login flow from within the TUI.
Instead, it tells you to quit, run `codex login`, and then run `codex`
again.
* `codex exec` does read from `auth.json` yet, nor does it direct the
user to go through the login flow if `OPENAI_API_KEY` is not be found.
* The `maybeRedeemCredits()` function from `get-api-key.tsx` has not
been ported from TypeScript to `login_with_chatgpt.py` yet:


https://github.com/openai/codex/blob/a67a67f3258fc21e147b6786a143fe3e15e6d5ba/codex-cli/src/utils/get-api-key.tsx#L84-L89

**Implementation**

Currently, the OAuth flow requires running a local webserver on
`127.0.0.1:1455`. It seemed wasteful to incur the additional binary cost
of a webserver dependency in the Rust CLI just to support login, so
instead we implement this logic in Python, as Python has a `http.server`
module as part of its standard library. Specifically, we bundle the
contents of a single Python file as a string in the Rust CLI and then
use it to spawn a subprocess as `python3 -c
{{SOURCE_FOR_PYTHON_SERVER}}`.

As such, the most significant files in this PR are:

```
codex-rs/login/src/login_with_chatgpt.py
codex-rs/login/src/lib.rs
```

Now that the CLI may load `OPENAI_API_KEY` from the environment _or_
`CODEX_HOME/auth.json`, we need a new abstraction for reading/writing
this variable, so we introduce:

```
codex-rs/core/src/openai_api_key.rs
```

Note that `std::env::set_var()` is [rightfully] `unsafe` in Rust 2024,
so we use a LazyLock<RwLock<Option<String>>> to store `OPENAI_API_KEY`
so it is read in a thread-safe manner.

Ultimately, it should be possible to go through the entire login flow
from the TUI. This PR introduces a placeholder `LoginScreen` UI for that
right now, though the new `codex login` subcommand introduced in this PR
should be a viable workaround until the UI is ready.

**Testing**

Because the login flow is currently implemented in a standalone Python
file, you can test it without building any Rust code as follows:

```
rm -rf /tmp/codex_home && mkdir /tmp/codex_home
CODEX_HOME=/tmp/codex_home python3 codex-rs/login/src/login_with_chatgpt.py
```

For reference:

* the original TypeScript implementation was introduced in
https://github.com/openai/codex/pull/963
* support for redeeming credits was later added in
https://github.com/openai/codex/pull/974

codex-rs/core/src/lib.rs

@@ -27,6 +27,7 @@ mod model_provider_info;
 pub use model_provider_info::ModelProviderInfo;
 pub use model_provider_info::WireApi;
 mod models;
+pub mod openai_api_key;
 mod openai_tools;
 mod project_doc;
 pub mod protocol;

codex-rs/core/src/model_provider_info.rs

@@ -11,6 +11,7 @@ use std::collections::HashMap;
 use std::env::VarError;
 
 use crate::error::EnvVarError;
+use crate::openai_api_key::get_openai_api_key;
 
 /// Wire protocol that the provider speaks. Most third-party services only
 /// implement the classic OpenAI Chat Completions JSON schema, whereas OpenAI
@@ -52,20 +53,27 @@ impl ModelProviderInfo {
     /// cannot be found, returns an error.
     pub fn api_key(&self) -> crate::error::Result<Option<String>> {
         match &self.env_key {
-            Some(env_key) => std::env::var(env_key)
-                .and_then(|v| {
-                    if v.trim().is_empty() {
-                        Err(VarError::NotPresent)
-                    } else {
-                        Ok(Some(v))
-                    }
-                })
-                .map_err(|_| {
-                    crate::error::CodexErr::EnvVar(EnvVarError {
-                        var: env_key.clone(),
-                        instructions: self.env_key_instructions.clone(),
+            Some(env_key) => {
+                let env_value = if env_key == crate::openai_api_key::OPENAI_API_KEY_ENV_VAR {
+                    get_openai_api_key().map_or_else(|| Err(VarError::NotPresent), Ok)
+                } else {
+                    std::env::var(env_key)
+                };
+                env_value
+                    .and_then(|v| {
+                        if v.trim().is_empty() {
+                            Err(VarError::NotPresent)
+                        } else {
+                            Ok(Some(v))
+                        }
                     })
-                }),
+                    .map_err(|_| {
+                        crate::error::CodexErr::EnvVar(EnvVarError {
+                            var: env_key.clone(),
+                            instructions: self.env_key_instructions.clone(),
+                        })
+                    })
+            }
             None => Ok(None),
         }
     }

codex-rs/core/src/openai_api_key.rs

@@ -0,0 +1,24 @@
+use std::env;
+use std::sync::LazyLock;
+use std::sync::RwLock;
+
+pub const OPENAI_API_KEY_ENV_VAR: &str = "OPENAI_API_KEY";
+
+static OPENAI_API_KEY: LazyLock<RwLock<Option<String>>> = LazyLock::new(|| {
+    let val = env::var(OPENAI_API_KEY_ENV_VAR)
+        .ok()
+        .and_then(|s| if s.is_empty() { None } else { Some(s) });
+    RwLock::new(val)
+});
+
+pub fn get_openai_api_key() -> Option<String> {
+    #![allow(clippy::unwrap_used)]
+    OPENAI_API_KEY.read().unwrap().clone()
+}
+
+pub fn set_openai_api_key(value: String) {
+    #![allow(clippy::unwrap_used)]
+    if !value.is_empty() {
+        *OPENAI_API_KEY.write().unwrap() = Some(value);
+    }
+}