Make token budget context identity configurable (#36970)

## What changed

- Add a `features.token_budget.mode` setting with `thread` and `name` values.
- Default context-window metadata to the thread ID while allowing configurations to retain the agent name.
- Preserve the selected identity mode when applying model token-budget defaults and replaying locked configuration.

## Testing

- Cover configuration parsing and both identity formats in token-budget context messages.

GitOrigin-RevId: f6e6d2c49e09e70c0352e803bb65784ef77e5bee

codex-rs/core/src/session/config_lock.rs

@@ -272,6 +272,7 @@ mod tests {
         config.multi_agent_v2.subagent_developer_instructions =
             Some("Locked subagent developer instructions.".to_string());
         config.token_budget = Some(crate::config::TokenBudgetConfig {
+            mode: codex_features::TokenBudgetMode::Thread,
             reminder_threshold_tokens: Some(16_000),
             reminder_message_template: "Locked reminder: {n_remaining} tokens.".to_string(),
             guidance_message: Some("Locked context-window guidance.".to_string()),
@@ -371,6 +372,7 @@ mod tests {
             features.token_budget,
             Some(FeatureToml::Config(TokenBudgetConfigToml {
                 enabled: Some(true),
+                mode: Some(codex_features::TokenBudgetMode::Thread),
                 reminder_threshold_tokens: Some(16_000),
                 reminder_message_template: Some(
                     "Locked reminder: {n_remaining} tokens.".to_string()

codex-rs/core/src/session/mod.rs

@@ -3502,6 +3502,12 @@ impl Session {
                     session_source
                         .get_agent_path()
                         .unwrap_or_else(codex_protocol::AgentPath::root),
+                    turn_context
+                        .config
+                        .token_budget
+                        .as_ref()
+                        .map(|config| config.mode)
+                        .unwrap_or_default(),
                     auto_compact_window_ids.first_window_id,
                     auto_compact_window_ids.previous_window_id,
                     auto_compact_window_ids.window_id,

codex-rs/core/src/session/tests.rs

@@ -9105,7 +9105,12 @@ async fn build_initial_context_adds_multi_agent_v2_subagent_usage_hint_as_develo
     let turn_context_mut =
         Arc::get_mut(&mut turn_context).expect("thread settings should not be shared");
     turn_context_mut.session_source = session_source;
-    Arc::make_mut(&mut turn_context_mut.config)
+    let config = Arc::make_mut(&mut turn_context_mut.config);
+    config.token_budget = Some(crate::config::TokenBudgetConfig {
+        mode: codex_features::TokenBudgetMode::Name,
+        ..crate::config::TokenBudgetConfig::default()
+    });
+    config
         .features
         .enable(Feature::TokenBudget)
         .expect("test config should allow token budget");

codex-rs/core/src/session/token_budget.rs

@@ -13,11 +13,14 @@ pub(super) fn has_explicit_settings(config: &Config) -> bool {
         .get("features")
         .and_then(|features| features.get("token_budget"))
         .and_then(|token_budget| token_budget.as_table())
-        .is_some_and(|settings| settings.keys().any(|key| key != "enabled"))
-        || config
-            .token_budget
-            .as_ref()
-            .is_some_and(|token_budget| token_budget != &TokenBudgetConfig::default())
+        .is_some_and(|settings| settings.keys().any(|key| key != "enabled" && key != "mode"))
+        || config.token_budget.as_ref().is_some_and(|token_budget| {
+            token_budget
+                != &TokenBudgetConfig {
+                    mode: token_budget.mode,
+                    ..TokenBudgetConfig::default()
+                }
+        })
 }
 
 pub(super) fn apply_model_defaults(config: &mut Config, model_info: &ModelInfo) {
@@ -34,6 +37,11 @@ pub(super) fn apply_model_defaults(config: &mut Config, model_info: &ModelInfo)
     };
 
     let token_budget = TokenBudgetConfig {
+        mode: config
+            .token_budget
+            .as_ref()
+            .map(|token_budget| token_budget.mode)
+            .unwrap_or_default(),
         reminder_threshold_tokens: Some(model_defaults.reminder_threshold_tokens),
         reminder_message_template: model_defaults.reminder_message_template.clone(),
         guidance_message: Some(model_defaults.guidance_message.clone()),