initialize pool from scratch after TableGC.Close

[mhamza15]

Description

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 refactors the TableGC pool initialization by extracting it into a helper and ensures the pool can be reinitialized after being closed.

• Extracted connpool.NewPool calls into a new newPool helper.
• Added nil-check in Open() to recreate the pool if it was closed.
• Updated Close() to properly close and nil out the pool.

Comments suppressed due to low confidence (2)

go/vt/vttablet/tabletserver/gc/tablegc.go:716

• [nitpick] The helper function name newPool is generic. Consider renaming it to newTableGCPool for clarity in this context.

func newPool(env tabletenv.Env) *connpool.Pool {

go/vt/vttablet/tabletserver/gc/tablegc.go:176

• There are no tests covering the scenario where Open() is called after Close(). Add a unit test to verify the pool is reinitialized correctly.

if collector.pool == nil {