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/config/mod.rs

@@ -172,6 +172,7 @@ mod requirements;
 mod resolved_permission_profile;
 #[cfg(test)]
 mod schema;
+mod token_budget_startup;
 pub use auth_keyring::bootstrap_auth_config;
 pub use auth_keyring::resolve_bootstrap_auth_keyring_backend_kind;
 pub use codex_agent_roles::AgentRoleConfig;
@@ -197,6 +198,7 @@ pub use permissions::compile_permission_profile;
 pub(crate) use permissions::is_builtin_permission_profile_name;
 pub use permissions::resolve_permission_profile;
 pub(crate) use resolved_permission_profile::PermissionProfileState;
+pub use token_budget_startup::TokenBudgetStartupConfig;
 
 const DEFAULT_IGNORE_LARGE_UNTRACKED_DIRS: i64 = 200;
 const DEFAULT_IGNORE_LARGE_UNTRACKED_FILES: i64 = 10 * 1024 * 1024;
@@ -1053,6 +1055,8 @@ pub struct Config {
 
     /// Context-window token budget configuration, when enabled.
     pub token_budget: Option<TokenBudgetConfig>,
+    /// Runtime snapshot of configured token-budget preferences before startup activation.
+    pub token_budget_startup_config: Option<TokenBudgetStartupConfig>,
     /// Shared token budget for the root thread and its sub-agents.
     pub rollout_budget: Option<RolloutBudgetConfig>,
     /// Current-time reminder and clock tool configuration, when enabled.
@@ -4318,6 +4322,7 @@ impl Config {
             ghost_snapshot,
             multi_agent_v2,
             token_budget,
+            token_budget_startup_config: None,
             rollout_budget,
             current_time_reminder,
             sleep_tool_mode,

codex-rs/core/src/config/token_budget_startup.rs

@@ -0,0 +1,31 @@
+//! Keeps configured token-budget preferences separate from session startup activation.
+//! Fresh child sessions restore this snapshot before applying their starting model's defaults;
+//! history forks retain their parent's effective activation and the original snapshot.
+
+use super::Config;
+use super::ConstraintResult;
+use super::TokenBudgetConfig;
+use codex_features::Feature;
+
+/// Token-budget preferences before a session applies experimental or model-owned activation.
+#[derive(Debug, Clone, PartialEq)]
+pub struct TokenBudgetStartupConfig {
+    enabled: bool,
+    token_budget: Option<TokenBudgetConfig>,
+}
+
+impl Config {
+    /// Captures configured preferences, removing any inherited startup activation first.
+    pub(crate) fn prepare_token_budget_for_startup(&mut self) -> ConstraintResult<()> {
+        if let Some(snapshot) = self.token_budget_startup_config.as_ref() {
+            self.features
+                .set_enabled(Feature::TokenBudget, snapshot.enabled)?;
+            self.token_budget = snapshot.token_budget.clone();
+        }
+        self.token_budget_startup_config = Some(TokenBudgetStartupConfig {
+            enabled: self.features.enabled(Feature::TokenBudget),
+            token_budget: self.token_budget.clone(),
+        });
+        Ok(())
+    }
+}