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.






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[//]: # (BEGIN SAPLING FOOTER)
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* #5803
* #5793
* __->__ #5658

codex-rs/protocol/src/protocol.rs

@@ -935,6 +935,7 @@ pub struct SessionMeta {
     pub instructions: Option<String>,
     #[serde(default)]
     pub source: SessionSource,
+    pub model_provider: Option<String>,
 }
 
 impl Default for SessionMeta {
@@ -947,6 +948,7 @@ impl Default for SessionMeta {
             cli_version: String::new(),
             instructions: None,
             source: SessionSource::default(),
+            model_provider: None,
         }
     }
 }