Fix kafkastreamer consumer group race conditions and backpressure handling

[Copilot]

The kafkastreamer adapter had race conditions during rebalances and unbuffered channels causing consumer evictions.

Changes

Ready channel race fix

• ConsumeClaim called multiple times during rebalances → replaced channel send with sync.Once

Iterator backpressure fix

• Unbuffered iterator channels blocked ConsumeClaim → added 100-element buffer
• Prevents rebalance timeout evictions under load

Event ID correction

• Changed from m.Offset + 1 to m.Offset for proper Kafka offset semantics

Code cleanup

• Removed duplicate unreachable select case in connector.go
• Renamed fixes_test.go → consumer_group_internal_test.go

// Before: race on rebalance
func (mp *msgProcessor) ConsumeClaim(...) {
    mp.ready <- true  // Multiple sends possible
    ...
}

// After: safe rebalance handling
func (mp *msgProcessor) ConsumeClaim(...) {
    mp.readyOnce.Do(func() { mp.ready <- true })
    ...
}

Applied to both msgProcessor and connectorProcessor. Tests verify concurrent ConsumeClaim calls and buffer capacity.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud