fix(logservice): create on-fly clients outside pool lock

[jiangxinmeng1]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #25275

What this PR does / why we need it:

This PR prevents clientPool.GetOnFly from holding the pool mutex while creating a new logservice client.

Previously, GetOnFly held c.cond.L during NewClient, which may perform network I/O and block for a long time when logservice replicas are unreachable. While that lock was held, normal pool operations such as Get, Put,
and GetWithWriteToken could not proceed, causing the WAL commit path to stall.

Changes:

• Move NewClient execution outside the clientPool mutex.
• Keep the lock only for checking whether the pool is closed.
• Re-check the closed state after client creation.
• Close the newly created client and return ErrClientPoolClosed if the pool was closed during creation.
• Add a regression test proving that pool Get is not blocked while GetOnFly is creating a client.

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[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-07-04 08:04 UTC · Rule: release-4.1 · triggered by rule Automatic queue on approval for release-4.1
• ✅ Checks skipped · PR is already up-to-date
• ✅ Merged — 2026-07-04 08:04 UTC · at da9b6d5b79017927308d61c63aec6382ce5c45ad · squash

This pull request spent 11 seconds in the queue, including 1 second running CI.

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