perf: optimize was_applied fast path for known LWT statements

[mykaul]

Summary

• Add a fast path in ResultSet.was_applied that skips batch detection when the query has a known LWT status from the server PREPARE response
• For BoundStatement queries where is_lwt() returns True, the batch_regex match and isinstance checks are entirely avoided
• The slow path (isinstance + regex) is preserved for BatchStatement, SimpleStatement batches, and non-LWT queries

Motivation

ResultSet.was_applied previously always ran batch detection on every call: two isinstance checks plus a regex match against the query string. For the most common LWT case (prepared INSERT/UPDATE IF via BoundStatement), the driver already knows from the server PREPARE response whether it's an LWT statement. Leveraging is_lwt() lets us skip the batch detection entirely in this fast path.

This is part of the LWT prepared statement performance improvement effort documented in scylladb#751 (optimization B4).

Changes

cassandra/cluster.py - ResultSet.was_applied:

• Cache self.response_future.query in a local variable (avoids repeated attribute lookups)
• Add fast path: if query.is_lwt() is True and it's not a BatchStatement, skip batch detection and go directly to single-row validation + result extraction
• Slow path is unchanged for BatchStatement, SimpleStatement, and non-LWT queries

tests/unit/test_resultset.py:

• test_was_applied_lwt_fast_path — tests the fast path with known LWT queries (all row factories, applied/not-applied, too-many-rows error)
• test_was_applied_non_lwt_fallback — tests that non-LWT SimpleStatement correctly falls through to slow path
• test_was_applied_batch_statement — tests that BatchStatement uses slow path with [applied] column validation

Testing

All unit tests pass:

• tests/unit/test_resultset.py — 17/17 passed (14 original + 3 new)
• tests/unit/test_query.py — 6/6 passed
• tests/unit/test_parameter_binding.py — 37/37 passed