Gate experimental context by model capability at session startup (#43147)

## Why

Experimental context activation previously checked the provider and account eligibility without checking model support. Child sessions also inherited token-budget activation from their parent, even when starting fresh with a different model.

## What changed

- Add `ModelInfo.supports_experimental_context`, defaulting to `false`, and enable it for the bundled `gpt-6-astra` model. Require this capability when activating experimental context.
- Snapshot configured token-budget preferences before startup activation. Restore them for fresh child sessions before applying their starting model's defaults, while history forks retain their parent's activation.
- Pass unresolved token-budget preferences to child sessions so they can use their own model's prompts.

## Testing

Extend coverage for unsupported models, model-switch guidance under explicit and experimental activation, and child configuration from both active and inactive parents. Verify that omitted capability metadata defaults to `false`.

GitOrigin-RevId: 02df9e171682267232fa923d5ea3f7af36527808

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

@@ -687,11 +687,22 @@ impl Session {
             .get_model_info(model.as_str(), &config.to_models_manager_config())
             .await;
         let auth = auth_manager.auth_cached();
-        token_budget::apply_experimental_context(Arc::make_mut(&mut config), auth.as_ref())?;
-        // Intentionally resolve `enabled` and `use_history_notes_extension` only at
-        // thread startup. Both activation flags stay fixed for this thread runtime,
-        // even if the selected model changes later.
-        token_budget::apply_model_defaults(Arc::make_mut(&mut config), &model_info);
+        // Forked subagents keep their parent's activation with the copied history.
+        // Fresh children restore configured preferences before applying startup defaults.
+        let inherits_token_budget = matches!(&conversation_history, InitialHistory::Forked(_))
+            && config.token_budget_startup_config.is_some();
+        if !inherits_token_budget {
+            Arc::make_mut(&mut config)
+                .prepare_token_budget_for_startup()
+                .map_err(|err| CodexErr::InvalidRequest(err.to_string()))?;
+            // Resolve activation for this runtime, including when resuming saved history.
+            token_budget::apply_experimental_context(
+                Arc::make_mut(&mut config),
+                auth.as_ref(),
+                &model_info,
+            )?;
+            token_budget::apply_model_defaults(Arc::make_mut(&mut config), &model_info);
+        }
         let configured_config = Arc::clone(&config);
         let multi_agent_version = config.multi_agent_version_override().or_else(|| {
             resolve_multi_agent_version(&conversation_history, inherited_multi_agent_version)

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

@@ -21,9 +21,11 @@ fn experimental_context_is_eligible(auth_mode: AuthMode, plan_type: Option<PlanT
 pub(super) fn apply_experimental_context(
     config: &mut Config,
     auth: Option<&CodexAuth>,
+    starting_model: &ModelInfo,
 ) -> std::io::Result<()> {
     let provider = &config.model_provider;
     if !config.features.enabled(Feature::ContextManagement)
+        || !starting_model.supports_experimental_context
         || !provider.supports_codex_backend_routes()
         || !provider.requires_openai_auth
         || provider.env_key.is_some()