docs: Add changelog entry for tablet shutdown tracking and validation

[promptless]

Documents PR #19009, which adds a new tablet_shutdown_time field to the Tablet protobuf and validates tablet records before connection attempts. This reduces unnecessary connection attempts to shutdown tablets and eliminates log noise from failed connections to invalid addresses.

Trigger Events

• vitessio/vitess PR #19009: grpctmclient: validate tablet record on dial

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Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

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🔧 I noticed that some CI checks failed for this PR. I'm investigating the failures and will work on fixing them.

[promptless]

Promptless escalated the following message to you. If you respond, please remember to tag Promptless:

I've investigated the CI failure on PR #19037. The check is failing because the PR needs a component label that reflects the code changes being documented.

Since this PR documents tablet shutdown tracking and connection validation features, please add the "Component: Cluster management" label to the PR. This should resolve the CI check failure.

(I'm unable to add labels directly as my GitHub access is read-only.)

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