Deduplicate default suite attributes in preferSwiftTesting - #2699

preferSwiftTesting was appending --default-test-suite-attributes even when a suite already declared the same attribute, producing duplicate annotations (e.g. two @MainActor lines). This change makes attribute insertion idempotent for existing suite-level attributes.

• Rule behavior update

  • Filter defaultTestSuiteAttributes against attributes already present on the XCTest suite declaration before insertion.
  • Preserve configured defaults that are missing; skip only duplicates.

• Regression coverage

  • Added a test for an XCTest suite already annotated with @MainActor while defaultTestSuiteAttributes: ["@MainActor"] is set.
  • Expected output now keeps a single @MainActor on the converted Swift Testing suite.

// Before (incorrect)
@MainActor
@MainActor
final class MyFeatureTests { ... }

// After
@MainActor
final class MyFeatureTests { ... }