[v2-11-test] Fix root logger level cache invalidation in LoggerMutationHelper

[seoin0110]

Why

Currently, the code assigns the log level directly to the .level attribute.
This violates the official Python documentation, which explicitly states:

• Note: Do not set this attribute directly - always use setLevel(), which has checks for the level passed to it.

This direct assignment bypasses the Python logging module's _invalidate_cache() mechanism. If the root logger was previously initialized with a different level (e.g., WARNING), the stale cache persists. This causes logs (e.g., INFO) to be suppressed during task execution even after the level update.

What

Change the assignment to use the standard method .setLevel().
This ensures the internal cache is cleared via _invalidate_cache() and the new level takes effect immediately.

[boring-cyborg]

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[jscheffl]

FYI tests fail on the same problem that I attempt to fix in #61651 ... probably unrelated to this PR.