feat: expose error and retry messages to SDK consumers

[cpfiffer]

Summary

• Add SDKErrorMessage and SDKRetryMessage types to the SDK message union
• Handle type=error and type=retry wire messages in transformMessage() instead of silently dropping them
• Export both new types from the SDK entry point

Previously, when the Letta API returned errors (rate limiting, auth failures, etc.), the SDK dropped the type=error wire message that contained the actual error detail (message, stop_reason, api_error). Consumers only saw the subsequent type=result with the opaque string "error" as both its error and stopReason fields. Similarly, type=retry messages with attempt counts and backoff timing were silently discarded.

Now both message types flow through to consumers, enabling meaningful error display instead of "Agent run failed: error [error]".

Test plan

• bun test -- 47/47 pass
• bun run build -- clean
• Tested end-to-end against a 429 rate-limit error on lettabot (Telegram channel). Error message now surfaces actual API error detail to the bot's stream loop.

Written by Cameron and Letta Code

"There are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies, and the other way is to make it so complicated that there are no obvious deficiencies." -- C.A.R. Hoare