feat: switch knowhere IO init to unified context pool API

[CLiqing]

Summary

• add backend-agnostic SetIOContextPool(size_t) entrypoint in knowhere config
• route disk IO pool initialization through unified IOContextPool API instead of AIO-specific path
• keep compatibility by preserving existing config behavior while validating the new init path in unit tests

Test plan

• Build knowhere in vecTool integration workspace (make engine=cardinal)
• Verify runtime search flow still works (runVectool.sh --data=siftsmall --stage=search --index_type=HNSW --index_version=9)
• Run full upstream knowhere CI suite

🤖 Generated with Claude Code

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[mergify]

@CLiqing 🔍 Important: PR Classification Needed!

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1. If you're fixing a bug, label it as kind/bug.
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[mergify]

@CLiqing e2e jenkins job failed, comment /run-e2e can trigger the job again.

[mergify]

@CLiqing e2e jenkins job failed, comment /run-e2e-sse can trigger the job again.

[alexanderguzhva]

@CLiqing please rebase