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/thread-manager-sample/src/main.rs

@@ -15,6 +15,7 @@ use codex_core_api::Arg0DispatchPaths;
 use codex_core_api::AskForApproval;
 use codex_core_api::AuthCredentialsStoreMode;
 use codex_core_api::AuthManager;
+use codex_core_api::AutoCompactTokenLimitScope;
 use codex_core_api::CodexThread;
 use codex_core_api::Config;
 use codex_core_api::ConfigLayerStack;
@@ -168,6 +169,7 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         review_model: None,
         model_context_window: None,
         model_auto_compact_token_limit: None,
+        model_auto_compact_token_limit_scope: AutoCompactTokenLimitScope::Total,
         model_provider_id,
         model_provider,
         personality: None,