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/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;
       }