test(dx): add client parity tests for Dictation and LLM Gateway

[pylrn]

Description

Extends tests/unit/test_dx.py to cover the developer-experience (DX) conventions for the newly added DictationTranscriber, AsyncDictationTranscriber, LLMGateway, and AsyncLLMGateway clients:

1. api_key= construction:
   • DictationTranscriber and LLMGateway accept an explicit api_key= without a global key set.
   • AsyncDictationTranscriber and AsyncLLMGateway accept api_key=, manage their own client, and clean it up via aclose().
2. api_key= precedence over caller-provided client:
   • Parametrizes all sync clients (Transcriber, SyncTranscriber, DictationTranscriber, LLMGateway) to verify that passing api_key= derives a new client while leaving the caller's client untouched.
   • Parametrizes all async clients (AsyncTranscriber, AsyncSyncTranscriber, AsyncDictationTranscriber, AsyncLLMGateway) to verify the same precedence, ownership flag, and cleanup semantics.
3. **Async client ownership semantics (aclose())**:
   • Verifies that passing a pre-existing AsyncClient without api_key= leaves that client open when aclose() is invoked across AsyncTranscriber, AsyncSyncTranscriber, AsyncDictationTranscriber, and AsyncLLMGateway.
4. Config type guards:
   • Verifies cross-product config rejections (DictationTranscriber rejecting TranscriptionConfig and SyncTranscriptionConfig, and prerecorded / sync transcribers rejecting DictationConfig) with descriptive type error messages.
5. Version bump:
   • Increments __version__ to 1.6.1 to satisfy CI version bump check.

Validation

• All 81 tests in tests/unit/test_dx.py pass.
• Entire test suite (pytest tests/unit) passes: 734 passed.