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/protocol.rs

@@ -275,6 +275,10 @@ pub enum EventMsg {
     /// Agent has completed all actions
     TaskComplete(TaskCompleteEvent),
 
+    /// Token count event, sent periodically to report the number of tokens
+    /// used in the current session.
+    TokenCount(TokenUsage),
+
     /// Agent text output message
     AgentMessage(AgentMessageEvent),
 
@@ -322,6 +326,15 @@ pub struct TaskCompleteEvent {
     pub last_agent_message: Option<String>,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize, Default)]
+pub struct TokenUsage {
+    pub input_tokens: u64,
+    pub cached_input_tokens: Option<u64>,
+    pub output_tokens: u64,
+    pub reasoning_output_tokens: Option<u64>,
+    pub total_tokens: u64,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize)]
 pub struct AgentMessageEvent {
     pub message: String,