Replace the pool when reopening.

[mhamza15]

Description

This is a backport of vitessio#18530

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[Copilot]

Pull Request Overview

This PR backports changes that replace the connection pool when reopening after closure. The primary goal is to ensure that pools are properly reset to a clean state when reopened, preventing potential issues with stale connections or corrupted pool state.

• Stores pool configuration and stats exporter references to enable pool recreation
• Replaces the ConnPool instance with a new one during the Close() operation
• Refactors stats registration to use a separate StatsExporter that can be reassigned to new pool instances

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
go/vt/vttablet/tabletserver/connpool/pool.go	Stores config and statsExporter, recreates ConnPool on close
go/vt/dbconnpool/connection_pool.go	Similar changes to store config/statsExporter and recreate pool on close
go/pools/smartconnpool/pool_test.go	Simplifies test assertions for pool closing behavior
go/pools/smartconnpool/pool.go	Adds pool state management and refactors stats registration

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