staleread, session: internal write request should be accepted with external ts (#39967)

[ti-chi-bot]

This is an automated cherry-pick of #39967

Signed-off-by: YangKeao yangkeao@chunibyo.icu

What problem does this PR solve?

Issue Number: close #39966

What is changed and how it works?

Allow writing queries for internal write request with tidb_enable_external_ts_read on.

Check List

Tests

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

Release note

Fix the issue that internal writing SQLs are refused with `tidb_enable_external_ts_read` on.

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick, it must first be approved by the collaborators.

AFTER it has been approved by collaborators, please ping the release team in a comment to request a cherry pick review.

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[wuhuizuo]

/hold high priority for trunk branch

[ti-chi-bot]

/unhold

[ti-chi-bot]

@ti-chi-bot: The following test 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	777b76a	link	true	/test unit-test

Full PR test history. Your PR dashboard.

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