core: add configurable <context_window_guidance> message (#29936)

## Why

This PR adds a configurable `<context_window_guidance>` developer
section immediately after `<context_window>`. Harness integrations need
this section to give the model deployment-specific instructions for
preparing for context-window transitions.

## What changed

- Add an optional `features.token_budget.guidance_message` config with a
1,000-byte runtime cap and generated schema support.
- Render configured guidance as a developer `ContextualUserFragment`
wrapped in `<context_window_guidance>` immediately after
`<context_window>`.
- Omit the section when guidance is unset, empty, or whitespace-only.
- Preserve the resolved value in config locks and classify persisted
guidance as contextual developer content.
- Add integration coverage for rendered content and ordering.

codex-rs/features/src/feature_configs.rs

@@ -88,6 +88,10 @@ pub struct TokenBudgetConfigToml {
     #[serde(skip_serializing_if = "Option::is_none")]
     #[schemars(length(min = 1, max = 1000))]
     pub reminder_message_template: Option<String>,
+    /// Guidance appended to the context-window metadata in a developer message.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    #[schemars(length(max = 1000))]
+    pub guidance_message: Option<String>,
 }
 
 impl FeatureConfig for TokenBudgetConfigToml {