fix(pubsub): avoid panics on Channel conflict and Ring empty subscribe

[VedantMadane]

Summary

Fixes #3761.

• PubSub.Channel / ChannelWithSubscriptions mutual-exclusion misuse no longer panics; returns a closed channel and logs a warning.
• Ring.Subscribe / PSubscribe / SSubscribe no longer panic on empty channels or shard lookup failure; return a PubSub with a sticky error (as the existing TODO suggested).

Test plan

• TestRingSubscribeEmptyChannelsNoPanic
• TestPubSubChannelMutualExclusionNoPanic

---

Note

Medium Risk
Changes PubSub failure semantics from panic to errors/closed channels; callers that relied on panics or expected blocking on misuse may see different behavior, though it is generally safer for production.

Overview
Fixes #3761 by replacing several PubSub panics with recoverable behavior.

Ring Subscribe / PSubscribe / SSubscribe no longer panic when channels are missing or shard lookup fails (e.g. empty ring). They return a PubSub built via failedPubSub that carries a stickyErr, which conn() and Receive* surface as errors instead of crashing at construction.

PubSub.Channel and ChannelWithSubscriptions now handle two failure modes without panicking: sticky construction errors, and calling the “wrong” channel API after the other was already used. Both paths log a warning and return an already-closed Go channel (logging uses context.Background() to avoid a race on c.cmd).

Tests cover Ring empty subscribe, channel API conflicts, and Ginkgo cases for sticky-error channels.

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[ndyakov]

@VedantMadane thank you for this, Ring is not officially supported or recommended anymore, but I will take a look when possible.

[ndyakov]

Thanks for tackling this — approach looks right and matches the TODO/issue, and keeping the signatures non-breaking is the right call. A couple of things before merge, main ones being a dead stickyErr write in Close() and the Ring sticky-error path hanging silently when consumed via Channel(). Details inline.

[VedantMadane]

Addressed in dc6f17e:

• Dropped the redundant stickyErr write in Close()
• Added short-circuiting in Channel() and ChannelWithSubscriptions() when stickyErr != nil && !closed to log a warning and return an already-closed channel (so for range pubsub.Channel() completes without hanging).
• Added warning logging in Ring.failedPubSub for visibility when constructing a failed PubSub.
• Updated docstrings for Subscribe/PSubscribe/SSubscribe and Channel/ChannelWithSubscriptions.
• Removed .gitattributes.
• Rebased on latest master.