Resolve token budgets from each step's active model (#41162)

## Why

Model settings can change between steps in the same turn. Token-budget defaults,
context-window limits, and model-visible guidance need to stay aligned with the
model captured for each step.

## What changed

- Preserve the turn's original token-budget preferences and resolve model-owned
  defaults when each `StepContext` is created.
- Use the step's model to determine context-window availability and token-budget
  guidance, while keeping explicit user settings unchanged.
- Emit one-time replacement or removal notices when context-window guidance
  changes, including compatibility with previously persisted world-state
  snapshots.
- Add `ModelInfo::usable_context_window()` to distinguish reserved-headroom
  capacity from the resolved context window and auto-compaction limit.

## Testing

- Cover mid-turn model switches with model defaults, explicit settings, config
  reloads, missing context windows, and missing destination guidance.
- Cover unchanged, replaced, removed, blank, and legacy guidance transitions.

GitOrigin-RevId: 91100a9f3625bef8a79ecd158eb028051c309eee

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

@@ -3319,6 +3319,11 @@ impl Session {
         // Capture once before asynchronous planning; all request consumers
         // retain this immutable settings version even if the turn is updated.
         let settings = turn_context.current_settings.load_full();
+        let token_budget = token_budget::resolve_token_budget(
+            turn_context.configured_token_budget.as_ref(),
+            turn_context.use_model_token_budget_defaults,
+            settings.model_info.as_ref(),
+        );
         let session_telemetry = settings.telemetry(&turn_context.session_telemetry);
         // Keep selections fixed for the turn while allowing their startup work to finish.
         let environments = turn_context.environments.refresh_readiness();
@@ -3402,6 +3407,7 @@ impl Session {
         .await??;
         Ok(Arc::new(StepContext {
             settings,
+            token_budget,
             session_telemetry,
             turn: turn_context,
             environments,