[Ktor] Add MustBeClosed annotation to buildKtor method (#1829)

Add @MustBeClosed annotation to buildKtor method

This commit adds the `@MustBeClosed` annotation to the `buildKtor` method in `HttpClientBuilder.kt` to indicate that the returned `HttpClient` instance must be closed by the caller. It also updates the test in `HttpClientBuilderTest.kt` to use the `use` function to ensure proper resource management.