[DO NOT MERGE / test bundle] proto: add paging_size_bytes to coprocessor.Request

[YuhaoZhang00]

Draft / test-bundle PR for the rc-precharge-classic-test integration bundle (classic / non-Premium variant of the RC paging pre-charge feature).

Summary

• Adds paging_size_bytes (field 17) to coprocessor.Request. When set, TiKV stops scanning when accumulated scanned bytes reach this limit, enabling byte-budget paging for finer-grained Resource Control.

Sibling PRs (test bundle)

• tikv/pd rc-precharge-classic-test
• tikv/client-go rc-precharge-classic-test
• pingcap/tidb rc-precharge-classic-test
• tikv/tikv rc-precharge-classic-test

This branch exists for upstream CI / cross-repo integration testing. Not intended to be merged as-is.

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