- Add close() method to HttpMockSequence class
- Implements no-op pattern consistent with HttpMock.close()
- Fixes AttributeError when using HttpMockSequence with context manager
- Add 5 comprehensive unit tests covering:
  * Method existence and callability
  * No-op behavior and multiple calls safety
  * Return value consistency (None)
  * Parity with HttpMock.close()
  * Usability after close()

All tests pass (5/5 in 0.33s)

Fixes googleapis#2359

Root cause: Commit 98888da (Sep 2020) added close() to HttpMock but
forgot to add it to HttpMockSequence. This PR completes that implementation.