[codex-analytics] report compaction analytics details (#26680)

## Why

Compaction analytics adds retained image count and compaction summary
output tokens for v1.5 specifically.

## What changed

- Add nullable `retained_image_count` and `compaction_summary_tokens`
fields to `codex_compaction_event`.
- Populate them only for `responses_compaction_v2`: retained images come
from the retained v2 compacted history, and summary tokens come from
`response.completed.token_usage.output_tokens`.
- Leave local and legacy remote compaction events as `null` for these
detail fields.

## Verification

- `just fmt`
- `just fix -p codex-core`
- `just test -p codex-core
build_v2_compacted_history_counts_retained_input_images`
- `git diff --check`

codex-rs/core/src/compact.rs

@@ -150,7 +150,7 @@ async fn run_compact_task_inner(
                     sess.as_ref(),
                     CompactionStatus::Interrupted,
                     Some(error),
-                    /*active_context_tokens_before*/ None,
+                    CompactionAnalyticsDetails::default(),
                 )
                 .await;
             return Err(CodexErr::TurnAborted);
@@ -174,7 +174,7 @@ async fn run_compact_task_inner(
                     sess.as_ref(),
                     status,
                     error,
-                    /*active_context_tokens_before*/ None,
+                    CompactionAnalyticsDetails::default(),
                 )
                 .await;
             return Err(CodexErr::TurnAborted);
@@ -185,7 +185,7 @@ async fn run_compact_task_inner(
             sess.as_ref(),
             status,
             error,
-            /*active_context_tokens_before*/ None,
+            CompactionAnalyticsDetails::default(),
         )
         .await;
     result.map(|_| ())
@@ -335,6 +335,13 @@ pub(crate) struct CompactionAnalyticsAttempt {
     start_instant: Instant,
 }
 
+#[derive(Clone, Copy, Default)]
+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>,
+}
+
 impl CompactionAnalyticsAttempt {
     pub(crate) async fn begin(
         sess: &Session,
@@ -363,8 +370,13 @@ impl CompactionAnalyticsAttempt {
         sess: &Session,
         status: CompactionStatus,
         error: Option<String>,
-        active_context_tokens_before: Option<i64>,
+        details: CompactionAnalyticsDetails,
     ) {
+        let CompactionAnalyticsDetails {
+            active_context_tokens_before,
+            retained_image_count,
+            compaction_summary_tokens,
+        } = details;
         let active_context_tokens_before =
             active_context_tokens_before.unwrap_or(self.active_context_tokens_before);
         let active_context_tokens_after = sess.get_total_token_usage().await;
@@ -382,6 +394,8 @@ impl CompactionAnalyticsAttempt {
                 error,
                 active_context_tokens_before,
                 active_context_tokens_after,
+                retained_image_count,
+                compaction_summary_tokens,
                 started_at: self.started_at,
                 completed_at: now_unix_seconds(),
                 duration_ms: Some(