Support compact SessionStart hooks (#21272)

# Why

Compaction replaces the live conversation history, so hooks that use
`SessionStart` to re-inject durable model context need a way to run
again after that rewrite.

Related - #19905 adds dedicated compact lifecycle hooks

# What

- add `compact` as a supported `SessionStart` source and matcher value
- change pending `SessionStart` state from a single slot to a small FIFO
queue so `resume` / `startup` / `clear` can be preserved alongside a
later `compact`
- drain all queued `SessionStart` sources before the next model request,
preserving their original order

# Testing

The new integration coverage verifies both the basic `compact` matcher
path and the stacked `resume` -> `compact` case where both hooks
contribute `additionalContext` to the next model turn.

codex-rs/core/src/session/session.rs

@@ -1184,7 +1184,7 @@ impl Session {
             Box::pin(sess.record_initial_history(initial_history)).await;
             {
                 let mut state = sess.state.lock().await;
-                state.set_pending_session_start_source(Some(session_start_source));
+                state.queue_pending_session_start_source(session_start_source);
             }
 
             Ok(sess)