feat: support session connect attrs in /slow_query (#1900)

[ti-chi-bot]

This is an automated cherry-pick of #1900

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]

@yibin87 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]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: shhdgit

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[yibin87]

/hold

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-06-11 00:57:08.753077083 +0000 UTC m=+1007929.823394463: ☑️ agreed by shhdgit.
• 2026-06-11 01:03:34.575138535 +0000 UTC m=+1008315.645455924: ✖️🔁 reset by yibin87.

[ti-chi-bot]

New changes are detected. LGTM label has been removed.