statistics: sync stats session time_zone with global value (#65821)

[ti-chi-bot]

This is an automated cherry-pick of #65821

What problem does this PR solve?

Issue Number: ref #65815

Problem Summary:
Root cause: internal stats sessions are created by getSessionFactoryInternal (pkg/session/session.go) which sets sessionVars.CommonGlobalLoaded = true, and internal SQL goes through ExecRestrictedSQL → ParseWithParams (pkg/session/session.go), which does not call loadCommonGlobalVariablesIfNeeded(). Together this means global sysvars like time_zone are never loaded for internal stats sessions, so they fall back to system TZ and skew time-based stats calculations (e.g. analyze_jobs duration).

What changed and how does it work?

• Ensure stats sessions explicitly load global time_zone in UpdateSCtxVarsForStats.
• Add a regression unit test to cover the time_zone skew scenario.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fix internal stats sessions to load @@time_zone from global settings, avoiding time offset in analyze job duration calculations.

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-chi-bot]

@0xPoe This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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Details

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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	ff94b9c	link	true	/test unit-test
idc-jenkins-ci-tidb/build	ff94b9c	link	true	/test build
idc-jenkins-ci-tidb/check_dev_2	ff94b9c	link	true	/test check-dev2
idc-jenkins-ci-tidb/check_dev	ff94b9c	link	true	/test check-dev
idc-jenkins-ci-tidb/mysql-test	ff94b9c	link	true	/test mysql-test

Full PR test history. Your PR dashboard.

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