feat: add manual and remote_v2 tags to compaction metric (#24608)

## Why
`codex.task.compact` only distinguished `local` vs `remote`, which made
it hard to answer simple counter questions in Statsig. Manual `/compact`
and automatic compaction were collapsed together, and the legacy remote
path was also collapsed with `remote_compaction_v2`.

## What Changed
- route `codex.task.compact` through a shared helper in
`core/src/tasks/mod.rs`
- add a `manual=true|false` tag so manual and automatic compaction can
be counted separately
- split the remote tag into `remote` and `remote_v2`
- emit the metric from the inline auto-compaction path in
`core/src/session/turn.rs` as well as the manual `CompactTask` path in
`core/src/tasks/compact.rs`
- add focused unit coverage for the new tag shapes in
`core/src/tasks/mod_tests.rs`

## Verification
- added unit coverage in `core/src/tasks/mod_tests.rs` covering manual
`remote_v2` tags and automatic `local` tags

codex-rs/core/src/tasks/mod.rs

@@ -62,6 +62,7 @@ pub(crate) use user_shell::UserShellCommandTask;
 pub(crate) use user_shell::execute_user_shell_command;
 
 const GRACEFULL_INTERRUPTION_TIMEOUT_MS: u64 = 100;
+const TASK_COMPACT_METRIC: &str = "codex.task.compact";
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub(crate) enum InterruptedTurnHistoryMarker {
@@ -145,6 +146,18 @@ fn emit_turn_memory_metric(
     );
 }
 
+pub(crate) fn emit_compact_metric(
+    session_telemetry: &SessionTelemetry,
+    compact_type: &'static str,
+    manual: bool,
+) {
+    session_telemetry.counter(
+        TASK_COMPACT_METRIC,
+        /*inc*/ 1,
+        &[("type", compact_type), ("manual", bool_tag(manual))],
+    );
+}
+
 fn bool_tag(value: bool) -> &'static str {
     if value { "true" } else { "false" }
 }