raft: Fix the max concurrency of IngestSST (relase-8.5)

[JaySon-Huang]

cherry-pick of #10609

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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

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CalvinNeo, JinheLin

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• OWNERS [CalvinNeo,JinheLin]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-12-12 02:40:32.075407357 +0000 UTC m=+1181576.889184929: ☑️ agreed by CalvinNeo.
• 2025-12-12 02:45:14.170064804 +0000 UTC m=+1181858.983842375: ☑️ agreed by JinheLin.