FEAT: Deprecate use_entra_auth and add auto-detect auth for Azure Speech converters

[varunj-msft]

Description

Deprecates the use_entra_auth parameter across both Azure Speech converters and the scorer pass-through chain, replacing it with auto-detection logic that matches the OpenAI target pattern.

Auth is now resolved automatically:

• String azure_speech_key (or AZURE_SPEECH_KEY env var) -> API key auth
• Callable azure_speech_key (sync or async token provider) -> Entra token auth via aad#{resource_id}#{token} format
• Neither provided -> automatic Entra ID auth via DefaultAzureCredential + azure_speech_resource_id

The deprecated use_entra_auth param is kept with a DeprecationWarning (removal target: v0.15.0) for backward compatibility.

Note: Both converters now enforce keyword-only args (*) per style guide. All existing callers already use keyword args, but this is technically a breaking change for positional usage. Also, azure_speech_key is no longer visible in the backend converter catalog since its type changed from Optional[str] to Union[str, Callable, None] - users configure the key via the AZURE_SPEECH_KEY env var in the backend.

Files changed:

• azure_speech_text_to_audio_converter.py - primary target
• azure_speech_audio_to_text_converter.py - identical auth logic
• audio_transcript_scorer.py, audio_true_false_scorer.py, audio_float_scale_scorer.py - deprecate pass-through param
• .env_example - updated comment

Tests and Documentation

• 26/26 converter unit tests pass (was 11 before; added tests for auto-detect, callable providers, async providers, deprecation warnings, env var fallback, missing credential errors)
• 963 passed, 8 skipped on broader unit suite (0 regressions)
• Ruff check clean, pre-commit hooks pass (ruff format, mypy strict)
• Doc notebooks (doc/code/converters/2_audio_converters.py/.ipynb) don't use use_entra_auth - no JupyText changes needed