redis: discard readonly pipeline connections

[NVGreg]

Outcome

Fixes REDIS_CLOSE_CONNECTION_ON_READONLY_ERROR=true so Redis READONLY replies inside pipelined commands also cause the pooled connection to be discarded and re-dialed.

Impact

Deployments that fail over by repointing an address at the new master can recover stale pooled connections even when the failing write is part of the normal INCRBY/EXPIRE pipeline. The failing operation still returns its READONLY error to the caller; only connection handling changes.

Scope

• Adds a read-only-aware pipeline action used only when REDIS_CLOSE_CONNECTION_ON_READONLY_ERROR is enabled.
• Leaves default flag-off behavior on native radix.NewPipeline().
• Preserves ordinary per-command Redis errors as connection-usable.
• Covers single/sentinel pipelined writes and cluster grouped pipelined writes.
• Updates README/settings docs to state that pipelined commands are covered.

Remaining gate

This is Draft for maintainer review. It was intentionally based on older commit 8fe6ea421048bdb2a8873d5f676b3f6eac76a997 to keep the branch push-compatible with the current GitHub token permissions. It should be reviewed with #1255 as compatible but separate work; #1255 handles closing connections on context cancellation, while this PR handles READONLY replies inside pipelines.

Compatibility with #1255

Locally merged this branch with #1255 (tmp/pr-1255-close-on-cancel) without conflicts. Focused Redis tests and race tests passed on the composed branch.

Verification

go test ./src/redis -run 'Test(ReadOnlyAwarePipelineDelegatesDoToEmbeddedConn|ReadOnly|PoolDiscardsConnOnReadOnlyPipeline|ExecuteGroupedPipelineDiscardsConnOnReadOnly)' -count=1
go test ./src/redis -run '^$' -bench BenchmarkNewPipelineAppendTwoActions -benchmem -count=3
go test ./src/redis ./test/redis -count=1
go test -race ./src/redis -run 'Test(ReadOnlyAwarePipeline|PoolDiscardsConnOnReadOnlyPipeline|ExecuteGroupedPipelineDiscardsConnOnReadOnly|ExecuteGroupedPipeline)' -count=1
go test ./...
git diff --check 8fe6ea42..HEAD

Composed with #1255:

go test ./src/redis -run 'Test(ReadOnly|PoolDiscardsConnOnReadOnlyPipeline|ExecuteGroupedPipelineDiscardsConnOnReadOnly|Close|Cancel)' -count=1
go test -race ./src/redis -run 'Test(ReadOnlyAwarePipeline|PoolDiscardsConnOnReadOnlyPipeline|ExecuteGroupedPipelineDiscardsConnOnReadOnly|Close|Cancel)' -count=1
git diff --check