refactor: dynamic ConversionRequest validation, remove 'or default' from adapters

- __post_init__: _apply_none_defaults() iterates dataclasses.fields() dynamically
- _NUMERIC_CONSTRAINTS and _ENUM_CONSTRAINTS dicts replace per-field if chains
- Both adapters pass values as-is (no 'or default' fallbacks)
- 18 validation tests + updated adapter tests for Enum assertions

abogen/pyqt/conversion_adapter.py

@@ -60,28 +60,28 @@ def build_conversion_request_from_thread(thread: Any) -> ConversionRequest:
         direct_text=thread.file_name if is_direct_text else None,
         original_filename=original_filename,
         # TTS Settings
-        language=thread.lang_code or "a",
+        language=thread.lang_code,
         tts_provider="kokoro",  # PyQt uses Kokoro by default
-        voice=thread.voice or "M1",
+        voice=thread.voice,
         voice_profile=getattr(thread, "voice_profile", None),
-        speed=thread.speed or 1.0,
+        speed=thread.speed,
         use_gpu=thread.use_gpu,
-        supertonic_total_steps=getattr(thread, "supertonic_total_steps", None) or 5,
+        supertonic_total_steps=getattr(thread, "supertonic_total_steps", 5),
         # Output Format
-        output_format=thread.output_format or "wav",
-        subtitle_mode=thread.subtitle_mode or "Disabled",
+        output_format=thread.output_format,
+        subtitle_mode=thread.subtitle_mode,
         subtitle_format=getattr(thread, "subtitle_format", "srt"),
-        max_subtitle_words=getattr(thread, "max_subtitle_words", None) or 50,
+        max_subtitle_words=getattr(thread, "max_subtitle_words", 50),
         # Save Options
-        save_mode=thread.save_option or "save_next_to_input",
+        save_mode=thread.save_option,
         output_folder=output_folder,
         save_chapters_separately=getattr(thread, "save_chapters_separately", False),
         merge_chapters_at_end=getattr(thread, "merge_chapters_at_end", True),
         separate_chapters_format=getattr(thread, "separate_chapters_format", "wav"),
         save_as_project=getattr(thread, "save_as_project", False),
         # Timing
-        silence_between_chapters=getattr(thread, "silence_duration", None) or 2.0,
-        chapter_intro_delay=getattr(thread, "chapter_intro_delay", None) or 0.0,
+        silence_between_chapters=getattr(thread, "silence_duration", 2.0),
+        chapter_intro_delay=getattr(thread, "chapter_intro_delay", 0.0),
         # Content Processing
         replace_single_newlines=getattr(thread, "replace_single_newlines", False),
         read_title_intro=getattr(thread, "read_title_intro", False),