[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/core/src/context/mod.rs

@@ -23,6 +23,7 @@ mod realtime_end_instructions;
 mod realtime_start_instructions;
 mod realtime_start_with_instructions;
 mod subagent_notification;
+mod token_budget_context;
 mod turn_aborted;
 mod user_instructions;
 mod user_shell_command;
@@ -60,6 +61,8 @@ pub(crate) use realtime_end_instructions::RealtimeEndInstructions;
 pub(crate) use realtime_start_instructions::RealtimeStartInstructions;
 pub(crate) use realtime_start_with_instructions::RealtimeStartWithInstructions;
 pub(crate) use subagent_notification::SubagentNotification;
+pub(crate) use token_budget_context::TokenBudgetContext;
+pub(crate) use token_budget_context::TokenBudgetRemainingContext;
 pub(crate) use turn_aborted::TurnAborted;
 pub(crate) use user_instructions::UserInstructions;
 pub(crate) use user_shell_command::UserShellCommand;

codex-rs/core/src/context/token_budget_context.rs

@@ -0,0 +1,68 @@
+use super::ContextualUserFragment;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub(crate) struct TokenBudgetContext {
+    window_id: u64,
+    tokens_left: i64,
+}
+
+impl TokenBudgetContext {
+    pub(crate) fn new(window_id: u64, tokens_left: i64) -> Self {
+        Self {
+            window_id,
+            tokens_left,
+        }
+    }
+}
+
+impl ContextualUserFragment for TokenBudgetContext {
+    fn role(&self) -> &'static str {
+        "developer"
+    }
+
+    fn markers(&self) -> (&'static str, &'static str) {
+        Self::type_markers()
+    }
+
+    fn type_markers() -> (&'static str, &'static str) {
+        ("<token_budget>\n", "\n</token_budget>")
+    }
+
+    fn body(&self) -> String {
+        let window_id = self.window_id;
+        let tokens_left = self.tokens_left;
+        format!(
+            "Current context window {window_id}.\nYou have {tokens_left} tokens left in this context window."
+        )
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub(crate) struct TokenBudgetRemainingContext {
+    tokens_left: i64,
+}
+
+impl TokenBudgetRemainingContext {
+    pub(crate) fn new(tokens_left: i64) -> Self {
+        Self { tokens_left }
+    }
+}
+
+impl ContextualUserFragment for TokenBudgetRemainingContext {
+    fn role(&self) -> &'static str {
+        "developer"
+    }
+
+    fn markers(&self) -> (&'static str, &'static str) {
+        Self::type_markers()
+    }
+
+    fn type_markers() -> (&'static str, &'static str) {
+        ("<token_budget>\n", "\n</token_budget>")
+    }
+
+    fn body(&self) -> String {
+        let tokens_left = self.tokens_left;
+        format!("You have {tokens_left} tokens left in this context window.")
+    }
+}