Migrate TestNG Assertion (instance) to AssertJ

[juherr]

What's changed

Adds TestNgAssertionToAssertJ, wired into the TestNgToAssertj aggregate, to migrate TestNG's hard, instance-based assertion class org.testng.asserts.Assertion to static AssertJ assertThat(...). Because Assertion fails immediately (unlike SoftAssert), it maps to the static assertThat entry point — the same target as org.testng.Assert.

// before
Assertion assertion = new Assertion();
assertion.assertEquals("actual", "expected");   // TestNG order: actual, then expected

// after
assertThat("actual").isEqualTo("expected");      // the now-dead local instance is removed

The recipe

• Rewrites instance assertion calls to static assertThat(...), scoped by exact receiver type (TypeUtils.isOfClassType(..., "org.testng.asserts.Assertion")), so custom Assertion subclasses are left untouched and it stays disjoint from SoftAssert extends Assertion.
• A visitBlock pass removes the now-unused local Assertion instance once its calls are rewritten (single-cycle; only when the variable has no remaining references). Fields are left in place.
• Covered: assertEquals/assertNotEquals (TestNG (actual, expected) order, plus message → .as(...) and floating-point delta → isCloseTo(..., within(...))/isNotCloseTo(...)), assertTrue/assertFalse, assertNull/assertNotNull, assertSame/assertNotSame, assertEqualsNoOrder → containsExactlyInAnyOrder, and fail(...) → static Assertions.fail(...) (no-arg fail() → fail(""), matching JUnitFailToAssertJFail).

Shared helper

Extracts the duplicated isFloatingPointType into a package-private TestNgAsserts and points every caller at it: TestNgAssertEqualsToAssertThat, TestNgAssertNotEqualsToAssertThat, TestNgSoftAssertToAssertJ, and the new recipe.

Test change worth noting

TestNgToAssertJTest#hardAssertionIsNotChanged is removed: it asserted that the aggregate leaves a hard org.testng.asserts.Assertion instance unchanged, which is no longer true now that this recipe migrates it. Its replacement, assertionInstanceMigratedViaAggregate, pins the new behaviour, and customAssertionSubclassIsNotChanged keeps the subclass-is-left-alone guarantee.

Tests cover every overload branch, the dead-local removal (plus retention-when-referenced and field-not-removed), and the fail variants.

[timtebeek]

Thanks again! With a slight bit of polish I'll merge as soon as CI passes.