fix(xai): self-heal the realtime chat context when an item's anchor is missing

[thecarlmartin]

Summary

Fixes an unrecoverable conversation-state cascade on livekit-plugins-xai realtime calls (filed as #6391).

On an interruption, truncate() deletes items. xAI's conversation.item.deleted event arrives with an empty item_id, so _handle_conversion_item_deleted guesses which item was removed (the first pending delete future). When several deletes are in flight the guess can pick the wrong item, desyncing the local RemoteChatContext from xAI's server-side list. After that, a server conversation.item.added referencing a still-present-server item finds no local anchor, RemoteChatContext.insert() raises previous_item_id … not found, and the base handler drops the item — which strands every later item that anchors to it. The whole context cascades into nothing and the model stalls (multi-second silences, then hard Item not found API errors).

In one ~2:16 trace the context broke ~69s in and never recovered:

WARN failed to insert item `f6dce9e1…`: previous_item_id `55273a67…` not found
WARN failed to insert item `f8cccbcc…`: previous_item_id `f6dce9e1…` not found   # chains off the dropped f6dce9e1
WARN failed to insert item `c1581c9d…`: previous_item_id `f6dce9e1…` not found
...

Fix

Anchor a newly-added item to the current tail whenever its previous_item_id is missing from the local context, so a single desync self-heals (at worst one item is slightly out of order) instead of dropping the item and cascading:

prev_id = event.previous_item_id
if prev_id is None or self._remote_chat_ctx.get(prev_id) is None:
    prev_id = self._remote_chat_ctx._tail.item.id if self._remote_chat_ctx._tail else None
event.previous_item_id = prev_id

This keeps RemoteChatContext.insert() strict and is scoped to the xAI handler (which already fills a None previous_item_id from the tail). A more thorough fix would also make _handle_conversion_item_deleted identify the deleted item precisely instead of guessing, but that needs the deleted item's id, which xAI's event doesn't currently provide — so this hardens the recovery path regardless.

Tests

Adds two tests to tests/test_realtime/test_xai_realtime_model.py:

• test_item_added_with_missing_anchor_self_heals_to_tail — an item whose anchor is absent is kept and lands at the tail (previously dropped).
• test_item_added_missing_anchor_does_not_cascade — a following item that anchors to the self-healed item also inserts (the cascade is broken).

All tests in that file pass locally. I don't have access to run the live-xAI integration suite, so a maintainer verification against the real API would be appreciated.

Fixes #6391.

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