This is an automated cherry-pick of #10609

What problem does this PR solve?

Issue Number: close #10605

Problem Summary: as the issue comment describe #10605 (comment)

KVStore::preHandleSSTsToDTFiles is the common path for both PreHandleSnapshot and IngestSST, calling prehandling_trace.waitForSubtaskResources will block the max concurrency of IngestSST by the pre handle snapshot thread-pool

What is changed and how it works?

* Parse `raftstore.apply-low-priority-pool-size` in `KVStore::fetchProxyConfig`
* `KVStore::getMaxPrehandleSubtaskSize` return different max concurrency according to the job_type

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  confirm that tiflash can successful parse and get the ingest sst max concurrency from tiflash-proxy

[2025/12/11 16:12:20.053 +08:00] [INFO] [KVStore.cpp:144] ["Parsed proxy config: snap_handle_pool_size=16 apply_low_priority_pool_size=72 engine_addr=10.2.12.81:9581"] [thread_id=1]

Run BR restore chbenchmark-1500

• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Fix an issue where TiFlash could not fully utilize more than 16 CPU cores during BR restore