[codex-analytics] report cached input tokens for v2 compaction (#27103)

## Summary

- add nullable `cached_input_tokens` to the compaction analytics event
- populate it from response usage for compaction v2
- leave it `null` for other compaction implementations

This adds visibility into prompt-cache usage for v2 compaction without
changing compaction behavior.

## Testing

- `just test -p codex-analytics`
- `just test -p codex-core
collect_compaction_output_accepts_additional_output_items`

codex-rs/analytics/src/analytics_client_tests.rs

@@ -1283,6 +1283,7 @@ fn compaction_event_serializes_expected_shape() {
                 active_context_tokens_after: 18_000,
                 retained_image_count: None,
                 compaction_summary_tokens: None,
+                cached_input_tokens: None,
                 started_at: 100,
                 completed_at: 106,
                 duration_ms: Some(6543),
@@ -1333,6 +1334,7 @@ fn compaction_event_serializes_expected_shape() {
                 "active_context_tokens_after": 18000,
                 "retained_image_count": null,
                 "compaction_summary_tokens": null,
+                "cached_input_tokens": null,
                 "started_at": 100,
                 "completed_at": 106,
                 "duration_ms": 6543
@@ -1855,6 +1857,7 @@ async fn compaction_event_ingests_custom_fact() {
                     active_context_tokens_after: 131_000,
                     retained_image_count: None,
                     compaction_summary_tokens: None,
+                    cached_input_tokens: None,
                     started_at: 100,
                     completed_at: 101,
                     duration_ms: Some(1200),
@@ -2781,6 +2784,7 @@ async fn subagent_events_use_inherited_connection_unless_turn_connection_is_expl
                     active_context_tokens_after: 64_000,
                     retained_image_count: None,
                     compaction_summary_tokens: None,
+                    cached_input_tokens: None,
                     started_at: 100,
                     completed_at: 101,
                     duration_ms: Some(1200),

codex-rs/analytics/src/events.rs

@@ -767,6 +767,7 @@ pub(crate) struct CodexCompactionEventParams {
     pub(crate) active_context_tokens_after: i64,
     pub(crate) retained_image_count: Option<usize>,
     pub(crate) compaction_summary_tokens: Option<i64>,
+    pub(crate) cached_input_tokens: Option<i64>,
     pub(crate) started_at: u64,
     pub(crate) completed_at: u64,
     pub(crate) duration_ms: Option<u64>,
@@ -1004,6 +1005,7 @@ pub(crate) fn codex_compaction_event_params(
         active_context_tokens_after: input.active_context_tokens_after,
         retained_image_count: input.retained_image_count,
         compaction_summary_tokens: input.compaction_summary_tokens,
+        cached_input_tokens: input.cached_input_tokens,
         started_at: input.started_at,
         completed_at: input.completed_at,
         duration_ms: input.duration_ms,

codex-rs/analytics/src/facts.rs

@@ -413,6 +413,7 @@ pub struct CodexCompactionEvent {
     pub active_context_tokens_after: i64,
     pub retained_image_count: Option<usize>,
     pub compaction_summary_tokens: Option<i64>,
+    pub cached_input_tokens: Option<i64>,
     pub started_at: u64,
     pub completed_at: u64,
     pub duration_ms: Option<u64>,

codex-rs/core/src/compact.rs

@@ -343,6 +343,7 @@ pub(crate) struct CompactionAnalyticsDetails {
     pub(crate) active_context_tokens_before: Option<i64>,
     pub(crate) retained_image_count: Option<usize>,
     pub(crate) compaction_summary_tokens: Option<i64>,
+    pub(crate) cached_input_tokens: Option<i64>,
 }
 
 impl CompactionAnalyticsAttempt {
@@ -379,6 +380,7 @@ impl CompactionAnalyticsAttempt {
             active_context_tokens_before,
             retained_image_count,
             compaction_summary_tokens,
+            cached_input_tokens,
         } = details;
         let active_context_tokens_before =
             active_context_tokens_before.unwrap_or(self.active_context_tokens_before);
@@ -399,6 +401,7 @@ impl CompactionAnalyticsAttempt {
                 active_context_tokens_after,
                 retained_image_count,
                 compaction_summary_tokens,
+                cached_input_tokens,
                 started_at: self.started_at,
                 completed_at: now_unix_seconds(),
                 duration_ms: Some(

codex-rs/core/src/compact_remote_v2.rs

@@ -281,6 +281,7 @@ async fn run_remote_compact_task_inner_impl(
     if let Some(token_usage) = token_usage {
         analytics_details.active_context_tokens_before = Some(token_usage.input_tokens);
         analytics_details.compaction_summary_tokens = Some(token_usage.output_tokens);
+        analytics_details.cached_input_tokens = Some(token_usage.cached_input_tokens);
     }
     let (compacted_history, retained_images) =
         build_v2_compacted_history(&prompt_input, compaction_output);