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

codex-rs/app-server-protocol/src/protocol.rs

@@ -327,6 +327,12 @@ pub struct ListConversationsParams {
     /// Opaque pagination cursor returned by a previous call.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub cursor: Option<String>,
+    /// Optional model provider filter (matches against session metadata).
+    /// - None => filter by the server's default model provider
+    /// - Some([]) => no filtering, include all providers
+    /// - Some([...]) => only include sessions with one of the specified providers
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub model_providers: Option<Vec<String>>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
@@ -338,6 +344,8 @@ pub struct ConversationSummary {
     /// RFC3339 timestamp string for the session start, if available.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub timestamp: Option<String>,
+    /// Model provider recorded for the session (resolved when absent in metadata).
+    pub model_provider: String,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]