Validate stored consumer offset range

[hubcio]

Prevent storing consumer offsets beyond the partition's actual message range. Applies to both set consumer offset and set consumer group offset — both flow through store_consumer_offset, which resolves the PollingConsumer (Consumer or ConsumerGroup variant) before storing.

What to focus on:

• Validate offset before store_consumer_offset_base is called — that's the single entry point for both consumer and consumer group offsets
• Query partition stats (messages_count) to determine max valid offset
• Handle empty partitions gracefully
• Add integration tests for boundary cases (both consumer and consumer group paths)

Done when storing an out-of-range offset fails with a descriptive error for both consumer and consumer group offsets.

Open questions:

• Should we allow offset equal to max_offset + 1 (next message position)? Doer decides, reviewers evaluate
• Should validation be optional or always enforced? Enforced

[gomnitrix]

@hubcio Hi, could you please assign this issue to me? I’d love to work on it.