feat: annotate conversations with model_provider for filtering

[bolinfest]

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 with API key login.)

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:

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

Lines 334 to 337 in bcd64c7

	/// Opaque cursor to pass to the next call to continue after the last item.
	/// if None, there are no more items to return.
	#[serde(skip_serializing_if = "Option::is_none")]
	pub next_cursor: Option<String>,

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