feat: support session connect attrs in /slow_query

[yibin87]

Issue

Issue Number: close #1899

Summary

• expose session connect attrs as an optional column on the /slow_query list page and keep it unchecked by default
• add a Session Attributes tab on the slow query detail page and render the value as JSON when the schema provides the field
• keep backward compatibility by only surfacing the field when it exists in the slow query schema, while preserving the existing warnings tab fallback for older TiDB versions
• extend the mock slow query schema, fixtures, and integration assertions to cover Session_connect_attrs end to end

Testing

• git diff --check
• go test ./pkg/apiserver/slowquery -count=1
• pnpm -C ui build:lib

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: shhdgit

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• OWNERS [shhdgit]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-06-10 05:53:35.426561465 +0000 UTC m=+939316.496878856: ☑️ agreed by shhdgit.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #1901.
But this PR has conflicts, please resolve them!