Migrate AssertJ @Rule soft assertions to SoftAssertionsExtension

[timtebeek]

JUnitSoftAssertions and JUnitBDDSoftAssertions only report their collected failures from TestRule#apply, which JUnit Jupiter never calls. JUnit4to5Migration left those @Rule fields untouched, so where JUnit 4 survives on the classpath (transitively, or deliberately for Testcontainers) the migrated tests compile, go green, and assert nothing.

HandleExternalResourceRules doesn't cover them: both types implement SoftAssertionsRule, not ExternalResource.

This adds JUnitSoftAssertionsToSoftAssertionsExtension, wired into JUnit4to5Migration ahead of the ExternalResource catch-all. For an instance @Rule field of either type it:

• swaps @Rule for @InjectSoftAssertions,
• changes the type to SoftAssertions / BDDSoftAssertions,
• drops final and the initializer, which SoftAssertionsExtension requires (it rejects static or final fields and instantiates the provider itself),
• adds @ExtendWith(SoftAssertionsExtension.class) to the declaring class unless already present.

@Rule
public final JUnitSoftAssertions softly = new JUnitSoftAssertions();

becomes

@ExtendWith(SoftAssertionsExtension.class)
class SoftlyTest {
    @InjectSoftAssertions
    public SoftAssertions softly;
}

@ClassRule/static fields are left alone, as the extension can't inject those.

• Fixes JUnit 4 to 5 migration leaves @Rule JUnitSoftAssertions behind, silently disabling the assertions #1097