feat: expose turn-start metadata to extensions (#23688)

## Why

The goal extension needs more context when a turn starts than
`turn_store` alone provides.

In particular, goal accounting needs the stable turn id, the effective
collaboration mode, and the cumulative token-usage baseline captured at
turn start so it can:

- suppress goal accounting for plan-mode turns
- compute exact per-turn deltas from cumulative `total_token_usage`
snapshots instead of relying on the most recent usage event alone
- keep the extension-owned accounting path aligned with the host turn
lifecycle

## What

- extend `codex_extension_api::TurnStartInput` to expose `turn_id`,
`collaboration_mode`, and `token_usage_at_turn_start`
- pass the full `TurnContext` plus the captured token-usage baseline
through the turn-start lifecycle emission path
- initialize goal turn accounting from the turn-start baseline and
collaboration mode
- switch goal token accounting to compute deltas from cumulative
`total_token_usage` snapshots
- add coverage for the new turn-start lifecycle fields and for
goal-accounting baseline behavior

## Testing

- added `turn_start_lifecycle_exposes_turn_metadata_and_token_baseline`
in `codex-rs/core/src/session/tests.rs`
- added `ext/goal/tests/accounting.rs` coverage for baseline-aware goal
accounting and plan-mode suppression

codex-rs/ext/extension-api/src/contributors/turn_lifecycle.rs

@@ -1,9 +1,17 @@
+use codex_protocol::config_types::CollaborationMode;
+use codex_protocol::protocol::TokenUsage;
 use codex_protocol::protocol::TurnAbortReason;
 
 use crate::ExtensionData;
 
 /// Input supplied when the host starts a turn.
 pub struct TurnStartInput<'a> {
+    /// Stable host-owned turn identifier.
+    pub turn_id: &'a str,
+    /// Effective collaboration mode for this turn.
+    pub collaboration_mode: &'a CollaborationMode,
+    /// Total token usage snapshot captured when the turn started.
+    pub token_usage_at_turn_start: &'a TokenUsage,
     /// Store scoped to the host session runtime.
     pub session_store: &'a ExtensionData,
     /// Store scoped to this thread runtime.