[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/analytics/src/events.rs

@@ -758,6 +758,8 @@ pub(crate) struct CodexCompactionEventParams {
     pub(crate) error: Option<String>,
     pub(crate) active_context_tokens_before: i64,
     pub(crate) active_context_tokens_after: i64,
+    pub(crate) retained_image_count: Option<usize>,
+    pub(crate) compaction_summary_tokens: Option<i64>,
     pub(crate) started_at: u64,
     pub(crate) completed_at: u64,
     pub(crate) duration_ms: Option<u64>,
@@ -970,6 +972,8 @@ pub(crate) fn codex_compaction_event_params(
         error: input.error,
         active_context_tokens_before: input.active_context_tokens_before,
         active_context_tokens_after: input.active_context_tokens_after,
+        retained_image_count: input.retained_image_count,
+        compaction_summary_tokens: input.compaction_summary_tokens,
         started_at: input.started_at,
         completed_at: input.completed_at,
         duration_ms: input.duration_ms,