feat(pubsub): sharded Pub/Sub for cluster mode

[Aryex]

Summary

Adds sharded (cluster-mode) Pub/Sub — Part 4 of the Pub/Sub implementation. This exposes the SPUBLISH / SSUBSCRIBE / SUNSUBSCRIBE family so cluster clients can publish and subscribe on shard channels, plus a connect-time sharded: subscription option. Sharded messages are routed by the channel's slot, matching the other GLIDE clients.

Stacked PR. This targets alexl/pubsub-part3-advanced-subs (Part 3, #316), not main. It must be retargeted to main once Part 3 merges. The diff shown against main will include Part 3's commits until then.

Issue link

Sub task of #135 Pub/Sub Support — Part 4 of the implementation plan.

Changes

Before, the sharded verbs were documented stubs raising NotImplementedError and publish refused sharded: true. After:

• publish(message, channel, sharded: true) dispatches SPUBLISH (wire order <channel> <message>, Ruby signature (message, channel)). Left deliberately un-guarded and batchable — it works inside pipelined/multi, and glide-core rejects it in standalone.
• ssubscribe(*channels, timeout_ms:) / sunsubscribe(*channels, timeout_ms:) (blocking) and ssubscribe_lazy(*channels) / sunsubscribe_lazy(*channels) (non-blocking). Same empty-list asymmetry as the exact verbs: ssubscribe/ssubscribe_lazy reject an empty list, sunsubscribe/sunsubscribe_lazy with no args target all sharded channels.
• Connect-time pubsub: { subscriptions: { sharded: [...] } }.
• New Valkey#cluster_mode? predicate.

Cluster-mode enforcement (the one design decision): the four subscribe verbs and the connect-time sharded config are cluster-only and raise ArgumentError in standalone. The guard is a private validate_cluster_mode! reading cluster_mode? as a method — mirroring how validate_resp3! reads protocol, keeping the mixin decoupled from client ivars. publish(sharded: true) is intentionally not guarded, so it stays batchable in a pipeline (a cluster_mode? call there would break the batch); the core decides.

Limitations

• Sharded Pub/Sub requires cluster_mode: true and Valkey 7.0.0+.
• pubsub_shardchannels / pubsub_shardnumsub / get_subscriptions introspection is Part 5, not here.

Testing

• bundle exec rubocop — clean (126 files, no offenses).
• bundle exec rake test:unit — 417 tests, 0 failures (added ~30 sharded unit tests covering request type, arg order, to_s coercion, timeout parsing, the RESP3 sweep, empty-list rules, the cluster-mode guard, and publish(sharded:) batchability).
• bundle exec rake test:standalone — sharded delivery tests skip; the standalone rejection tests (verbs + config) pass.
• bundle exec rake test:cluster — all 9 sharded integration tests pass, including a cross-slot routing test (distinct hash tags, proving SPUBLISH routes by channel not subscriber node) and a batchable-pipeline test.

Pre-existing, unrelated cluster-suite failures were observed and are not from this change (no Pub/Sub tests among them): the lib_name/client_info_tag validation family (#246, stale native lib), TLS errors (TLS_CERT_DIR unset), and a flaky cluster_bumpepoch epoch-state assertion.

Checklist

• This Pull Request is related to an issue.
• Commit message describe your changes
• Commits are signed off (git commit -s) per the DCO.
• Tests are added or updated.
• CHANGELOG.md and documentation files are updated.
• Linters have been run (bundle exec rubocop) and pass.
• Destination branch is correct - main. (Stacked on alexl/pubsub-part3-advanced-subs; retarget to main after Part 3 merges.)