fix(information_schema): statistics INDEX_TYPE should use idx.algo instead of NULL

[ck89119]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #23929

What this PR does / why we need it:

information_schema.STATISTICS 视图中 INDEX_TYPE 字段被硬编码为 NULL，与 MySQL 行为不一致。

本 PR 将 INDEX_TYPE 改为使用 mo_indexes.algo 字段的值，与 SHOW INDEX 行为保持一致。

Changes:

• pkg/util/sysview/predefined.go: NULL AS INDEX_TYPE → idx.algo AS INDEX_TYPE
• pkg/bootstrap/versions/v3_0_0/tenant_upgrade_list.go: 添加 STATISTICS 视图升级 entry
• test/distributed/cases/ddl/index_metadata.sql: 添加 bvt 测试用例

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-03-23 12:26 UTC · Rule: release-3.0
• ✅ Checks passed · in-place
• ✅ Merged — 2026-03-23 13:15 UTC · at 97025621f35d5dff609e1be8f49822597fb8c72c

This pull request spent 49 minutes 13 seconds in the queue, including 49 minutes 1 second running CI.

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