Add body_after_prefix auto-compact token limit scope (#22870)

## Why

`model_auto_compact_token_limit` has only been able to budget the full
active context. That makes it hard to set a small "growth since
compaction" budget for sessions that preserve a large carried window
prefix: the preserved prefix can consume the whole budget and force
immediate repeated compaction.

This PR adds an opt-in `body_after_prefix` scope so callers can apply
`model_auto_compact_token_limit` to sampled output and later growth
after the current carried prefix, while still forcing compaction before
the full model context window is exhausted.

## What changed

- Adds `AutoCompactTokenLimitScope` with the existing `total` behavior
as the default and a new `body_after_prefix` mode:
[`config_types.rs`](https://github.com/openai/codex/blob/973806b1cb35792555bead994cb3ed94656eb171/codex-rs/protocol/src/config_types.rs#L24-L37).
- Threads `model_auto_compact_token_limit_scope` through config loading,
`Config`, `core-api`, and app-server v2 schema/TypeScript generation.
- Records the first observed input-token count for a `body_after_prefix`
compaction window and uses it as the baseline when deciding whether the
scoped auto-compaction budget is exhausted:
[`turn.rs`](https://github.com/openai/codex/blob/973806b1cb35792555bead994cb3ed94656eb171/codex-rs/core/src/session/turn.rs#L743-L781).
- Keeps a hard context-window cap in `body_after_prefix`, so scoped
budgeting cannot let the active context overrun the usable window.

## Verification

Added compact-suite coverage for the two key behaviors:
`body_after_prefix` does not re-compact just because the carried prefix
is larger than the scoped budget, and it still compacts when the total
active context reaches the configured context window:
[`compact.rs`](https://github.com/openai/codex/blob/973806b1cb35792555bead994cb3ed94656eb171/codex-rs/core/tests/suite/compact.rs#L3003-L3128).

codex-rs/app-server-protocol/src/protocol/v2/config.rs

@@ -3,6 +3,7 @@ use super::AskForApproval;
 use super::SandboxMode;
 use super::shared::default_enabled;
 use codex_experimental_api_macros::ExperimentalApi;
+use codex_protocol::config_types::AutoCompactTokenLimitScope;
 use codex_protocol::config_types::ForcedLoginMethod;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::config_types::Verbosity;
@@ -251,6 +252,7 @@ pub struct Config {
     pub review_model: Option<String>,
     pub model_context_window: Option<i64>,
     pub model_auto_compact_token_limit: Option<i64>,
+    pub model_auto_compact_token_limit_scope: Option<AutoCompactTokenLimitScope>,
     pub model_provider: Option<String>,
     #[experimental(nested)]
     pub approval_policy: Option<AskForApproval>,

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -1511,6 +1511,7 @@ fn config_granular_approval_policy_is_marked_experimental() {
         review_model: None,
         model_context_window: None,
         model_auto_compact_token_limit: None,
+        model_auto_compact_token_limit_scope: None,
         model_provider: None,
         approval_policy: Some(AskForApproval::Granular {
             sandbox_approval: false,
@@ -1551,6 +1552,7 @@ fn config_approvals_reviewer_is_marked_experimental() {
         review_model: None,
         model_context_window: None,
         model_auto_compact_token_limit: None,
+        model_auto_compact_token_limit_scope: None,
         model_provider: None,
         approval_policy: None,
         approvals_reviewer: Some(ApprovalsReviewer::AutoReview),
@@ -1585,6 +1587,7 @@ fn config_nested_profile_granular_approval_policy_is_marked_experimental() {
         review_model: None,
         model_context_window: None,
         model_auto_compact_token_limit: None,
+        model_auto_compact_token_limit_scope: None,
         model_provider: None,
         approval_policy: None,
         approvals_reviewer: None,
@@ -1641,6 +1644,7 @@ fn config_nested_profile_approvals_reviewer_is_marked_experimental() {
         review_model: None,
         model_context_window: None,
         model_auto_compact_token_limit: None,
+        model_auto_compact_token_limit_scope: None,
         model_provider: None,
         approval_policy: None,
         approvals_reviewer: None,