[core] Fix flaky column type update test

[ArnavBalyan]

Purpose

• SchemaManagerTest intentionally injects artificial failure to verify retry behavior.
• The existing column update test reads schema without retrying, this causes the injected ArtificialException to fail the tests randomly and CI failure.
• Fix by wrapping the schema reads with a retryArtificialException.

Tests

• UT

[QuakeWang]

+1