kafka-mesh: Add second cluster batching test (blueberries → cluster2)

[Copilot]

PR #1031 tests batched producing only to apricots (cluster1). To demonstrate the mesh filter's key behavior—splitting batched ProduceRequests across upstream clusters—add testing for a second topic routed to cluster2.

Changes

• example.rst: Extended Step 8 with blueberries topic example showing 20 batched messages to cluster2
• verify.sh: Added test sending 20 messages to blueberries, verifying all arrive at cluster2

Matches coverage of upstream test_producing_with_batched_records which validates cross-cluster batching with apricots → cluster1 and berries → cluster2.

Original prompt

Enhancement to PR #1031

The current PR #1031 adds documentation and tests for high-volume batched producing, but only tests sending to one topic (apricots → cluster1).

To fully cover the mesh filter's key behavior—splitting batched ProduceRequests across different upstream clusters—we need to also document and test sending batched messages to a second topic routed to a different cluster.

What to Add

1. kafka-mesh/example.rst - Extend Step 8

After the existing apricots example, add documentation showing:

• Sending 20 messages rapidly to a b-prefixed topic (e.g., blueberries) which routes to cluster2
• Consuming from cluster2 to verify all 20 messages arrived
• Explanation that this demonstrates the mesh filter correctly splitting batched requests by destination cluster

2. kafka-mesh/verify.sh - Add second topic test

After the existing apricots test, add:

• Send 20 messages to blueberries topic
• Consume from cluster2 and verify count is 20
• Assert success

Context

This is needed to match the coverage of the original envoy integration test test_producing_with_batched_records which sends 100 messages each to two topics (apricots → cluster1, berries → cluster2) with batching enabled, verifying both are correctly routed.

Files to Modify

1. kafka-mesh/example.rst - Extend the new Step 8 section
2. kafka-mesh/verify.sh - Add test for second topic/cluster

Style

Follow the existing patterns in the PR - use the same bash loop style, same verification approach, same doc formatting.

This pull request was created from Copilot chat.

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