Add item streaming events (#5546)

Adds AgentMessageContentDelta, ReasoningContentDelta,
ReasoningRawContentDelta item streaming events while maintaining
compatibility for old events.

---------

Co-authored-by: Owen Lin <owen@openai.com>

codex-rs/core/src/client_common.rs

@@ -197,6 +197,7 @@ fn strip_total_output_header(output: &str) -> Option<&str> {
 pub enum ResponseEvent {
     Created,
     OutputItemDone(ResponseItem),
+    OutputItemAdded(ResponseItem),
     Completed {
         response_id: String,
         token_usage: Option<TokenUsage>,
@@ -205,9 +206,6 @@ pub enum ResponseEvent {
     ReasoningSummaryDelta(String),
     ReasoningContentDelta(String),
     ReasoningSummaryPartAdded,
-    WebSearchCallBegin {
-        call_id: String,
-    },
     RateLimits(RateLimitSnapshot),
 }