Track prompt cache write token usage (#33454)

## What changed

- Parse `cache_write_tokens` from response input token details and carry it through token usage aggregation.
- Expose `cache_write_input_tokens` in protocol, app-server, exec, and TypeScript SDK usage events, defaulting it to zero for compatibility with older payloads.
- Report cache-write usage in analytics, metrics, traces, and rollout data alongside existing cache-read usage.

## Testing

- Add coverage for response parsing, usage event output, aggregation, analytics, metrics, and telemetry fields.

GitOrigin-RevId: 634e4fbe3086a8923c0ee13b622573e7d9d221bd

sdk/typescript/samples/basic_streaming.ts

@@ -56,7 +56,7 @@ const handleEvent = (event: ThreadEvent): void => {
       break;
     case "turn.completed":
       console.log(
-        `Used ${event.usage.input_tokens} input tokens, ${event.usage.cached_input_tokens} cached input tokens, ${event.usage.output_tokens} output tokens, ${event.usage.reasoning_output_tokens} reasoning output tokens.`,
+        `Used ${event.usage.input_tokens} input tokens, ${event.usage.cached_input_tokens} cached input tokens, ${event.usage.cache_write_input_tokens} cache write input tokens, ${event.usage.output_tokens} output tokens, ${event.usage.reasoning_output_tokens} reasoning output tokens.`,
       );
       break;
     case "turn.failed":

sdk/typescript/src/events.ts

@@ -23,6 +23,8 @@ export type Usage = {
   input_tokens: number;
   /** The number of cached input tokens used during the turn. */
   cached_input_tokens: number;
+  /** The number of input tokens written to the prompt cache during the turn. */
+  cache_write_input_tokens: number;
   /** The number of output tokens used during the turn. */
   output_tokens: number;
   /** The number of reasoning output tokens used during the turn. */

sdk/typescript/src/thread.ts

@@ -103,6 +103,8 @@ export class Thread {
         }
         if (parsed.type === "thread.started") {
           this._id = parsed.thread_id;
+        } else if (parsed.type === "turn.completed") {
+          parsed.usage.cache_write_input_tokens ??= 0;
         }
         yield parsed;
       }

sdk/typescript/tests/run.test.ts

@@ -37,6 +37,7 @@ describe("Codex", () => {
       ];
       expect(result.items).toEqual(expectedItems);
       expect(result.usage).toEqual({
+        cache_write_input_tokens: 0,
         cached_input_tokens: 12,
         input_tokens: 42,
         output_tokens: 5,

sdk/typescript/tests/runStreamed.test.ts

@@ -47,6 +47,7 @@ describe("Codex", () => {
         {
           type: "turn.completed",
           usage: {
+            cache_write_input_tokens: 0,
             cached_input_tokens: 12,
             input_tokens: 42,
             output_tokens: 5,