Refactor JUnit extensions to avoid using static fields.

[slawekjaranowski]

Static filed in Junit extension can impact on tests executed in parallel. For static methods preserver current test context in local thread.

(cherry picked from commit 52c4d43)

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[gnodet]

Clean cherry-pick of #11829 from master to maven-4.0.x. The refactoring correctly replaces static mutable fields (context, injector, basedir) with ThreadLocal + ExtensionContext.Store, making the JUnit extensions safe for parallel test execution.

Verified:

• ThreadLocal cleanup in afterEach() is in a finally block — no leak risk
• Store lifecycle is managed by JUnit automatically
• Ordering in MojoExtension.beforeEach() is correct — setContext() before any getContext() consumers
• Namespace isolation (MAVEN_DI_EXTENSION, MOJO_EXTENSION) is an improvement over GLOBAL
• Null check removal for basedir is safe — MavenDIExtension.getBasedir() always returns non-null
• Test updates in SimpleDITest match the new setupContainer(ExtensionContext) signature

CI green across all 22 checks (ubuntu/macos/windows, JDK 17/21/25).

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