[codex] Add token budget context feature (#27438)

## Why

The model should be able to see bounded context-window budget metadata
when the `token_budget` feature is enabled. The full-window message is
only injected with full context, while normal turns get a smaller
follow-up only when reported usage first crosses a budget threshold.

## What changed

- Added the `TokenBudget` feature flag.
- Added `<token_budget>` developer fragments for full context-window
metadata and current-window remaining tokens.
- Inserted the threshold message during normal turn handling by
comparing token usage before and after sampling, avoiding persistent
threshold bookkeeping.
- Added core integration coverage for full-context-only metadata and
25/50/75 percent threshold messages.

## Verification

- `just test -p codex-core token_budget`
- `git diff --check`

codex-rs/features/src/lib.rs

@@ -197,6 +197,8 @@ pub enum Feature {
     GuardianApproval,
     /// Enable persisted thread goals and automatic goal continuation.
     Goals,
+    /// Add current context-window metadata to model-visible context.
+    TokenBudget,
     /// Route MCP tool approval prompts through the MCP elicitation request path.
     ToolCallMcpElicitation,
     /// Prompt Codex Apps connector auth failures through MCP URL elicitations.
@@ -1142,6 +1144,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::TokenBudget,
+        key: "token_budget",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::CollaborationModes,
         key: "collaboration_modes",