refactor: remove redundant as-any casts in event-stream and agent

[oldschoola]

Summary

Removes 3 redundant as any casts across two packages:

packages/ai/src/utils/event-stream.ts

EventStream.end() and endWaiting() resolved waiting consumers with { value: undefined as any, done: true }. The as any cast is unnecessary — IteratorResult<T> is IteratorYieldResult<T> | IteratorReturnResult<TReturn> where TReturn defaults to any, so undefined is assignable to the value field when done: true.

packages/agent/src/agent.ts

The turn_end handler accessed errorMessage via (event.message as any).errorMessage. The errorMessage field is already on the AssistantMessage interface (packages/ai/src/types.ts:524), so the cast is unnecessary.

Verification

• bun check passes
• 81 tests pass across agent.test.ts, agent-loop.test.ts, yield.test.ts, proxy-stream-disconnect.test.ts

[roboomp]

P1: small, coherent cleanup with changelog coverage; bun check passed locally.
One should-fix: packages/ai/src/utils/event-stream.ts still leans on IteratorResult<T>'s implicit any return type instead of tightening the waiter result to undefined.
No duplicate/superseding PR found in the GitHub search results for this fix. Thanks for the cleanup.

[roboomp]

should-fix This still type-checks because waiting.resolve is typed as IteratorResult<T>, whose TReturn defaults to any. That removes the as any text but keeps the same implicit any escape hatch for both done: true resolves. Could we type the waiter result and the promise in [Symbol.asyncIterator] as IteratorResult<T, undefined> instead, so undefined is checked without relying on any?

[oldschoola]

Good point — addressed in the latest push. The waiter type and the Promise in the async iterator are now explicitly typed as IteratorResult<T, undefined> instead of the default IteratorResult<T, any>, so the undefined value is type-checked without relying on the implicit any for TReturn.