feat(session): allow stream message IDs

[theadriann]

Description

Adds optional messageId support to DroidSession.stream() by forwarding it through MessageOptions to the underlying add_user_message request.

Why

Some SDK consumers need a stable, client-generated id for the user message that starts a stream. Without this, they have to bypass DroidSession and call lower-level client APIs just to correlate later session notifications with the originating message.

SDK impact

This is additive. Existing callers keep the same behavior when messageId is omitted. When provided, the SDK includes it in add_user_message.params.messageId, matching the existing protocol field.

What this enables

Higher-level DroidSession users can track message identity for UI state, persistence, replay, and session operations without dropping down to DroidClient.addUserMessage().

Testing

• pnpm lint
• pnpm typecheck
• pnpm exec vitest run tests/session.test.ts
• pnpm format:check
• pnpm test
• pnpm build