gh-22729: Make StompSession.Subscription.unsubscribe() return Receiptable

[Songdoeon]

Motivation

Currently Subscription.unsubscribe() is a void method, so clients have no way to know when an UNSUBSCRIBE frame has actually been processed by the server. This PR brings unsubscribe() in line with send() and acknowledge() by returning a Receiptable that callers can use to register callbacks and track the receipt.

Summary of Changes

• Interface

  • StompSession.Subscription — both unsubscribe() overloads now return Receiptable instead of void.

• Implementation

  • DefaultStompSession.unsubscribe(String, StompHeaders) (private helper) now returns a Receiptable by creating a ReceiptHandler after adding a receipt-id header.
  • DefaultStompSession.DefaultSubscription.unsubscribe(...) now invokes the helper and simply returns its Receiptable.

• Tests (in DefaultStompSessionTests)

  • unsubscribeWithReceipt()
    Verifies that calling unsubscribe() returns a non‑null Receiptable and that the sent frame still only contains the subscription ID when auto‑receipt is off.
  • unsubscribeWithCustomHeaderAndReceipt()
    Verifies that with autoReceipt=true and custom headers, the UNSUBSCRIBE frame contains the subscription ID, the custom header, and the generated receipt header, and that the returned Receiptable holds the same receipt-id.
  • receiptReceivedOnUnsubscribe()
    Verifies that if a RECEIPT frame arrives for the receipt-id, a task registered on the returned Receiptable is executed.

All existing tests pass, and the new tests cover the added behavior.

Closes gh-22729