feat: annotate conversations with model_provider for filtering (#5658)

Because conversations that use the Responses API can have encrypted
reasoning messages, trying to resume a conversation with a different
provider could lead to confusing "failed to decrypt" errors. (This is
reproducible by starting a conversation using ChatGPT login and resuming
it as a conversation that uses OpenAI models via Azure.)

This changes `ListConversationsParams` to take a `model_providers:
Option<Vec<String>>` and adds `model_provider` on each
`ConversationSummary` it returns so these cases can be disambiguated.

Note this ended up making changes to
`codex-rs/core/src/rollout/tests.rs` because it had a number of cases
where it expected `Some` for the value of `next_cursor`, but the list of
rollouts was complete, so according to this docstring:


bcd64c7e72/codex-rs/app-server-protocol/src/protocol.rs (L334-L337)

If there are no more items to return, then `next_cursor` should be
`None`. This PR updates that logic.






---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/5658).
* #5803
* #5793
* __->__ #5658

codex-rs/core/tests/suite/cli_stream.rs

@@ -75,9 +75,17 @@ async fn chat_mode_stream_cli() {
     server.verify().await;
 
     // Verify a new session rollout was created and is discoverable via list_conversations
-    let page = RolloutRecorder::list_conversations(home.path(), 10, None, &[])
-        .await
-        .expect("list conversations");
+    let provider_filter = vec!["mock".to_string()];
+    let page = RolloutRecorder::list_conversations(
+        home.path(),
+        10,
+        None,
+        &[],
+        Some(provider_filter.as_slice()),
+        "mock",
+    )
+    .await
+    .expect("list conversations");
     assert!(
         !page.items.is_empty(),
         "expected at least one session to be listed"