[Bug] Validate every Lite consumer subscription against its bound topic

## Version

Reproduced on develop at `e3458616d`.

## Problem

`ClientProcessor.validateLiteSubTopic` validates only `subList.iterator().next()`. A Lite consumer group with a configured `liteBindTopic` can include another subscription for a different topic as long as the first set element matches.

## Reproduction

1. Configure a consumer group with `liteBindTopic=topic1`.
2. Build a subscription set whose first element targets `topic1` and whose second element targets `topic2`.
3. Call `validateLiteSubTopic`.
4. Observe that no `GrpcProxyException` is thrown.

## Expected

Every subscription in the set must match the configured bound topic; any mismatch must be rejected with `ILLEGAL_TOPIC`.

## Regression coverage

`ClientProcessorTest` contains a focused failing test with one matching and one mismatching subscription.

Start in ClientProcessor.validateLiteSubTopic and inspect the focused failing case in ClientProcessorTest. Run that test with a subscription set containing one matching and one mismatching topic. Done means every subscription is checked and any mismatch is rejected with GrpcProxyException and ILLEGAL_TOPIC.

Written by the indexing model from the issue text.