[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/analytics_client_tests.rs

@@ -1253,6 +1253,8 @@ fn compaction_event_serializes_expected_shape() {
                 error: None,
                 active_context_tokens_before: 120_000,
                 active_context_tokens_after: 18_000,
+                retained_image_count: None,
+                compaction_summary_tokens: None,
                 started_at: 100,
                 completed_at: 106,
                 duration_ms: Some(6543),
@@ -1301,6 +1303,8 @@ fn compaction_event_serializes_expected_shape() {
                 "error": null,
                 "active_context_tokens_before": 120000,
                 "active_context_tokens_after": 18000,
+                "retained_image_count": null,
+                "compaction_summary_tokens": null,
                 "started_at": 100,
                 "completed_at": 106,
                 "duration_ms": 6543
@@ -1821,6 +1825,8 @@ async fn compaction_event_ingests_custom_fact() {
                     error: Some("context limit exceeded".to_string()),
                     active_context_tokens_before: 131_000,
                     active_context_tokens_after: 131_000,
+                    retained_image_count: None,
+                    compaction_summary_tokens: None,
                     started_at: 100,
                     completed_at: 101,
                     duration_ms: Some(1200),
@@ -2743,6 +2749,8 @@ async fn subagent_thread_started_inherits_parent_connection_for_new_thread() {
                     error: None,
                     active_context_tokens_before: 131_000,
                     active_context_tokens_after: 64_000,
+                    retained_image_count: None,
+                    compaction_summary_tokens: None,
                     started_at: 100,
                     completed_at: 101,
                     duration_ms: Some(1200),