fix(sdk): include persisted IDs in agent message notifications

[matej21]

Summary

• Include the persisted chat message ID in agentMessage notifications from both tellUser and XML communication.
• Let clients reconcile live notifications with stored history without guessing from content or timestamps.
• Assert that notification IDs match the stored message IDs in both integration paths.

Compatibility

The notification gains a messageId field; persisted events and message IDs are unchanged. Existing consumers that ignore additional fields continue to work. Consumers constructing typed agentMessage payloads must include the field.

Verification

• 40 user-chat integration tests passed.
• Clean monorepo TypeScript build passed.
• SDK test typecheck passed.
• Biome lint and git diff --check passed.

No release or deployment performed.