Routing fixes

[rukai]

This PR includes 2 unrelated changes to routing:

1. prefetch required routing metadata for DeleteRecords and OffsetDelete.
   • This was missed in KafkaSinkCluster: Fix DeleteRecords routing #1803 and KafkaSinkCluster: route OffsetDelete #1802
   • Without this logic we would not be able to route these message types unless another message type was recently sent that relies on the same metadata. That is how the issue went unnoticed.
2. Change RequestSplitAndRouter::get_request_frame to return &mut Self::Request instead of Option<&mut Self::Request>
   • This does not change functionality or fix anything but makes it easier to catch mistakes
   • The caller ignored the value if it was None which made for a very tricky to track down issue due to a typo when implementing KafkaSinkCluster: route DescribeProducers #1814
   • This approach immediately panics if we pass in the wrong type which is good since it indicates an unrecoverable programmer error.

I dont think the performance regression is meaningful.
While it is a kafka benchmark it is purely benching the encoder/decoder which I didnt touch in this PR.

[codspeed-hq]

CodSpeed Performance Report

Merging #1815 will not alter performance

_{Comparing rukai:routing_fixes (9c8300a) with main (65ce7cb)}

Summary

✅ 38 untouched benchmarks