Keep V5 orderbook sorted and fix crash on delta delete of absent level

[massimiliano1991]

Addresses the two defects in _process_delta_orderbook reported in #369, scoped to the parts that are uncontroversial correctness fixes. It intentionally does not touch the delta→snapshot re-labelling, which the maintainer noted in #369 is the documented/intended behaviour — this PR leaves that exactly as-is.

What it fixes

1. Unsorted book. New price levels were appended and the side was never re-sorted. Bybit deltas can add levels anywhere in the range, so after a few insert deltas data[topic]["b"][0] / ["a"][0] stop being the best bid/ask. This PR re-sorts each side after applying a delta (bids high→low, asks low→high), so [0] is always the best level.

2. Crash on delete of an absent level. A qty-0 delta for a level not currently held made _helpers.find_index raise StopIteration (it wraps a bare next()), which propagated out of _process_normal_message and dropped the message. The delete branch now checks existence first (same pattern already used by the insert branch) and skips a missing level.

What it does not change

• The type="snapshot" re-labelling and the merged-book payload delivered to callbacks are untouched.
• find_index's signature/behaviour is untouched (the guard is at the call site).

Tests

Two regression tests added in tests/test_pybit.py:

• inserting levels inside the range keeps both sides sorted and [0] the best bid/ask;
• a delete of an absent level is a no-op instead of raising.

Full suite passes locally (pytest tests/test_pybit.py → 46 passed).

Refs #369