test(streaming): add extensive tests for StreamingHandler to enhance coverage (#1183)

* fix(tests): ensure proper asyncio task cleanup in test_streaming_handler.py

- modified the `StreamingConsumer` class in to store its asyncio task
- added a `cancel()` method, and handle `asyncio.CancelledError`
- Updating test functions (`test_single_chunk`,
  `test_sequence_of_chunks`) and the helper function
  (`_test_pattern_case`) to call `consumer.cancel()` in a
  `finally` block.

These changes prevent `RuntimeError: Event loop is closed` and
`Task was destroyed but it is pending!` warnings by ensuring
background tasks are correctly cancelled and awaited upon test
completion.

* test(streaming): add extensive tests for StreamingHandler to enhance
coverage

- Added tests for various functionalities of StreamingHandler, including:
  - Piping streams between handlers
  - Buffering enable/disable behavior
  - Handling multiple stop tokens
  - Metadata inclusion and processing
  - Suffix and prefix pattern handling
  - Edge cases for __anext__ method
  - First token handling and generation info
- Improved test coverage for async methods and error scenarios.
- Addressed potential issues with streaming termination signals.

Author: Pouyanpi