feat: show number of tokens remaining in UI (#1388)

When using the OpenAI Responses API, we now record the `usage` field for
a `"response.completed"` event, which includes metrics about the number
of tokens consumed. We also introduce `openai_model_info.rs`, which
includes current data about the most common OpenAI models available via
the API (specifically `context_window` and `max_output_tokens`). If
Codex does not recognize the model, you can set `model_context_window`
and `model_max_output_tokens` explicitly in `config.toml`.

When then introduce a new event type to `protocol.rs`, `TokenCount`,
which includes the `TokenUsage` for the most recent turn.

Finally, we update the TUI to record the running sum of tokens used so
the percentage of available context window remaining can be reported via
the placeholder text for the composer:

![Screenshot 2025-06-25 at 11 20
55 PM](https://github.com/user-attachments/assets/6fd6982f-7247-4f14-84b2-2e600cb1fd49)

We could certainly get much fancier with this (such as reporting the
estimated cost of the conversation), but for now, we are just trying to
achieve feature parity with the TypeScript CLI.

Though arguably this improves upon the TypeScript CLI, as the TypeScript
CLI uses heuristics to estimate the number of tokens used rather than
using the `usage` information directly:


296996d74e/codex-cli/src/utils/approximate-tokens-used.ts (L3-L16)

Fixes https://github.com/openai/codex/issues/1242

codex-rs/core/src/config.rs

@@ -10,6 +10,7 @@ use crate::config_types::UriBasedFileOpener;
 use crate::flags::OPENAI_DEFAULT_MODEL;
 use crate::model_provider_info::ModelProviderInfo;
 use crate::model_provider_info::built_in_model_providers;
+use crate::openai_model_info::get_model_info;
 use crate::protocol::AskForApproval;
 use crate::protocol::SandboxPolicy;
 use dirs::home_dir;
@@ -30,6 +31,12 @@ pub struct Config {
     /// Optional override of model selection.
     pub model: String,
 
+    /// Size of the context window for the model, in tokens.
+    pub model_context_window: Option<u64>,
+
+    /// Maximum number of output tokens.
+    pub model_max_output_tokens: Option<u64>,
+
     /// Key into the model_providers map that specifies which provider to use.
     pub model_provider_id: String,
 
@@ -234,6 +241,12 @@ pub struct ConfigToml {
     /// Provider to use from the model_providers map.
     pub model_provider: Option<String>,
 
+    /// Size of the context window for the model, in tokens.
+    pub model_context_window: Option<u64>,
+
+    /// Maximum number of output tokens.
+    pub model_max_output_tokens: Option<u64>,
+
     /// Default approval policy for executing commands.
     pub approval_policy: Option<AskForApproval>,
 
@@ -387,11 +400,23 @@ impl Config {
 
         let history = cfg.history.unwrap_or_default();
 
+        let model = model
+            .or(config_profile.model)
+            .or(cfg.model)
+            .unwrap_or_else(default_model);
+        let openai_model_info = get_model_info(&model);
+        let model_context_window = cfg
+            .model_context_window
+            .or_else(|| openai_model_info.as_ref().map(|info| info.context_window));
+        let model_max_output_tokens = cfg.model_max_output_tokens.or_else(|| {
+            openai_model_info
+                .as_ref()
+                .map(|info| info.max_output_tokens)
+        });
         let config = Self {
-            model: model
-                .or(config_profile.model)
-                .or(cfg.model)
-                .unwrap_or_else(default_model),
+            model,
+            model_context_window,
+            model_max_output_tokens,
             model_provider_id,
             model_provider,
             cwd: resolved_cwd,
@@ -687,6 +712,8 @@ disable_response_storage = true
         assert_eq!(
             Config {
                 model: "o3".to_string(),
+                model_context_window: Some(200_000),
+                model_max_output_tokens: Some(100_000),
                 model_provider_id: "openai".to_string(),
                 model_provider: fixture.openai_provider.clone(),
                 approval_policy: AskForApproval::Never,
@@ -729,6 +756,8 @@ disable_response_storage = true
         )?;
         let expected_gpt3_profile_config = Config {
             model: "gpt-3.5-turbo".to_string(),
+            model_context_window: Some(16_385),
+            model_max_output_tokens: Some(4_096),
             model_provider_id: "openai-chat-completions".to_string(),
             model_provider: fixture.openai_chat_completions_provider.clone(),
             approval_policy: AskForApproval::UnlessTrusted,
@@ -786,6 +815,8 @@ disable_response_storage = true
         )?;
         let expected_zdr_profile_config = Config {
             model: "o3".to_string(),
+            model_context_window: Some(200_000),
+            model_max_output_tokens: Some(100_000),
             model_provider_id: "openai".to_string(),
             model_provider: fixture.openai_provider.clone(),
             approval_policy: AskForApproval::OnFailure,