feat: allow injecting a custom connection pool via pool_factory

[wizzense]

Motivation

Deployments whose connection parameters are not static environment variables cannot use the settings-built pool:

• Sentinel-managed masters — the pool that follows failover is a SentinelConnectionPool built from Sentinel.master_for(...), which no REDIS_URL/KV setting can express.
• Secret-manager-issued passwords — the credential is fetched at startup (Vault, AWS SM, etc.), not present in the environment.

Today the only way in is reaching into the private _PoolState and assigning app.state._redis from a custom lifespan — which works, but pins consumers to internals (we are running exactly that in production against 0.8.0 and would like to stop).

Design

One public seam, no behavior change for existing users:

FastAPIRedis(app).lifespan(pool_factory=make_my_pool)   # sync or async callable

or, for plain FastAPI(lifespan=redis_lifespan) users:

app.state.redis_pool_factory = make_my_pool

• The factory is a zero-argument sync or async callable returning the redis.asyncio.ConnectionPool to use (async matters: fetching a credential at startup is I/O).
• The lifespan owns the returned pool and closes it on shutdown, identical to the settings-built one.
• Cluster mode raises ValueError rather than silently ignoring the override.
• Every caching / rate-limiting dependency is untouched — they already resolve through the pool state.

Tests

tests/unit/test_pool_injection.py: sync factory, async factory, state-attribute path, cluster refusal, and a regression guard for the default settings-built path. Full unit run: 108 passed.

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