feat: accept redis client in rate limit RedisBackend

[ComeBertrand]

Allow passing a pre-built redis.Redis client as an alternative to a URL. When a client is provided, its connection_pool is reused by the limits RedisStorage, consistent with other remoulade Redis backends.

[Copilot]

Pull request overview

Adds support for supplying a pre-built redis.Redis client to the rate limit RedisBackend, so the backend can reuse an existing connection_pool instead of always constructing storage from a URL—aligning rate-limit Redis behavior with other Remoulade Redis backends.

Changes:

• Extend remoulade.rate_limits.backends.RedisBackend to accept client and make url optional, raising when neither is provided.
• Add/adjust test fixtures to exercise rate limiting against stub, Redis-by-URL, and Redis-by-client backends.
• Add a unit test asserting RedisBackend() requires either url or client.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
remoulade/rate_limits/backends/redis.py	Accept optional client, optional url, and build limits Redis storage from a provided connection pool.
tests/conftest.py	Introduce fixtures for Redis rate-limit backends (URL/client) and parametrize rate_limit_backend.
tests/middleware/test_rate_limit.py	Add a test for the “url or client required” constructor validation.

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