Fix inconsistent ClassLoader usage between class presence check and actual class loading

[linw-bai]

Fix inconsistent ClassLoader usage between class presence check and actual class loading

Background

LettuceClassUtils#getDefaultClassLoader() currently returns the Thread Context ClassLoader (TCCL) when available.
However, LettuceClassUtils#isPresent(...) relies on this method to check class existence, while the actual class loading is later performed using the ClassLoader of LettuceClassUtils itself.
This leads to an inconsistent ClassLoader strategy:

• Presence check: uses Thread Context ClassLoader
• Actual loading: uses LettuceClassUtils.class.getClassLoader()
  In environments with multiple ClassLoaders (e.g. OSGi, Servlet containers, shaded applications, plugin systems), this can cause:
• isPresent(...) returning true
• Followed by a ClassNotFoundException during actual class loading
  This is a classic false-positive detection issue caused by mismatched ClassLoaders.

Root Cause

The root problem is that:

• getDefaultClassLoader() returns TCCL
• But Class.forName(...) and direct loading later use a different ClassLoader
  So the class existence check is not aligned with the loader that actually loads the class.

Solution

This PR makes the ClassLoader strategy consistent by:

• Ensuring that both class presence checks and real class loading use the same ClassLoader
• Preferring LettuceClassUtils.class.getClassLoader() as the primary loader

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[a-TODO-rov]

Hello @linw-bai
You need to make PR against the main branch first and then the fix will be backported to earlier versions if needed.
I am closing this PR for now. Feel free to open it against main and we will review it.