fix(redis): build ClusterPipeline span metadata from _execution_strategy in redis-py 6+ (#4728)

* fix(redis): build ClusterPipeline span metadata from _execution_strategy in redis-py 6+

redis-py 6 refactored ClusterPipeline so that queued commands are no
longer appended to `command_stack`; they now live on
`_execution_strategy.command_queue`. As a result the redis instrumentor
emitted ClusterPipeline spans with an empty DB_STATEMENT and a
`db.redis.pipeline_length` of 0.

`_build_span_meta_data_for_pipeline` now reads commands from
`_execution_strategy.command_queue` when present, falling back to
`command_stack` / `_command_stack` for non-cluster pipelines and older
redis-py versions.

Fixes #4084

* chore(redis): rename changelog fragment to PR number

* test(redis): add docker regression test for ClusterPipeline span metadata

Exercises a real RedisCluster pipeline and asserts the redis-py 6+ code
path (command_stack empty, _execution_strategy.command_queue populated)
is genuinely hit before checking the emitted span statement and
db.redis.pipeline_length. Guards against regression #4084.

* Bump redis-py to 6.4.0 in docker tests so the ClusterPipeline regression test exercises the redis-py 6+ code path

* Fix RediSearch import for redis-py 6+ in docker tests

redis-py 6.x renamed redis.commands.search.indexDefinition to
index_definition (snake_case). The camelCase module was removed, so
bumping redis to 6.4.0 broke collection of the entire redis functional
test module. IndexDefinition/IndexType are unchanged in the new module.

* fix(redis): read async ClusterPipeline commands from private _command_queue

The redis-py 6+ sync cluster strategy exposes queued commands via a public
command_queue property, but the async cluster strategy only exposes the
private _command_queue attribute. Reading only command_queue left the async
ClusterPipeline span with an empty statement and a default "redis" name once
the docker tests were bumped to redis-py 6.4.0. Resolve _command_queue as a
fallback so both sync and async cluster pipelines populate span metadata.

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Co-authored-by: Tammy Baylis <96076570+tammy-baylis-swi@users.noreply.github.com>
Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>
Co-authored-by: Lukas Hering <40302054+herin049@users.noreply.github.com>