Tolerate interior fs_events seq holes in EventBus gap detection

[mornyx]

Summary

EventBus.EventsSince previously treated any seq hole between the client cursor and the first returned event as "events were pruned" and forced a client reset:

if since+1 < firstSeq { return nil, headSeq, false }

That assumption is wrong in two cases:

1. Shared-schema fs_events tables (one table serving many tenants): other tenants' interleaved seqs make per-tenant holes the norm — every poll would detect a "gap" and reset continuously (reset storm).
2. Standalone tables: rolled-back inserts burn AUTO_INCREMENT ids, creating the same benign interior holes.

Pruning only ever happens at the left edge of the retained window, so a gap is now a reset condition only when the cursor falls behind the oldest retained seq (since+1 < oldest) — identical semantics as before for actual pruning, and interior holes are delivered through. The no-rows branch already used exactly this comparison; this PR makes the got-rows branch consistent with it and updates the doc comment.

This is a prerequisite for moving fs_events onto shared (fs_id-keyed) tables.

Test plan

• go test ./pkg/server/ -run 'TestEventBus' -count=1 -v — all pass
• New TestEventBusEventsSinceInteriorGapDelivers: burned middle seq → delivered, no reset (previously would have reset)
• New TestEventBusEventsSincePrunedGapResets: left-edge pruning past the cursor → still resets
• bin/golangci-lint run ./pkg/server/... — 0 issues

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Review details

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Configuration used: defaults

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📥 Commits

Reviewing files that changed from the base of the PR and between bfd070e and a0976cd.

📒 Files selected for processing (2)

• pkg/server/eventbus.go
• pkg/server/eventbus_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/fs-events-gap-tolerance

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