Migrate javaagent module unit tests to the unitTests suite

Is your feature request related to a problem? Please describe.

Javaagent modules contain tests that exercise helper or instrumentation code without requiring the agent. These tests currently live in configurations that remove agent classes from the classpath, which can lead to workarounds such as reflection or bespoke test setup.

Describe the solution you'd like

After #20059 introduces the unitTests suite convention:

• Audit javaagent modules throughout the codebase and migrate eligible tests to that suite.
• Look for tests using reflection only to reach code that would be directly accessible from the new unit-test configuration, and remove that reflection where possible.
• Update the relevant repository guidance and .github/agents/knowledge articles to encode the new patterns, including when tests belong in the unitTests suite and when direct access should be preferred over reflection.

Relevant examples and discussion:

• #19845: https://github.com/open-telemetry/opentelemetry-java-instrumentation/pull/19845#discussion_r3976412521
• #20011: https://github.com/open-telemetry/opentelemetry-java-instrumentation/pull/20011#discussion_r3979013437
• Convention introduced in #20059

Describe alternatives you've considered

Continue adding per-module test tasks or retain reflection-based workarounds. This would duplicate Gradle configuration and keep tests harder to read and maintain.

Additional context

The audit should distinguish true agent integration tests from ordinary unit tests so existing agent coverage remains intact. Any guidance updates should capture patterns discovered during the audit so future instrumentation and reviews apply them consistently.

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