refactor: extract synthesize_text() domain function (#2)

Combines TTSContext.normalize() + run_tts_segment_loop() into a single
domain function. Both UIs call synthesize_text() instead of inlining
normalize → TTS loop. UI-specific concerns (provider resolution,
progress display, cancellation) stay in the UI layer.

Tests: 1253 passed

abogen/pyqt/conversion.py

@@ -36,7 +36,7 @@ from abogen.domain.output_paths import (
 )
 from abogen.domain.audio_helpers import build_ffmpeg_command, to_float32
 from abogen.domain.audio_sink import AudioSink, open_audio_sink
-from abogen.domain.conversion_engine import run_tts_segment_loop, SegmentStats, SegmentInfo
+from abogen.domain.conversion_engine import synthesize_text, SegmentStats, SegmentInfo
 from abogen.domain.intro_outro import resolve_intro, resolve_outro
 from abogen.domain.audio_buffer import (
     create_silence,
@@ -982,14 +982,6 @@ class ConversionThread(QThread):
                         print("Using split pattern: (unprintable)")
 
                     for text_segment in text_segments:
-                        # Normalize text before TTS
-                        try:
-                            text_segment = self._tts_context.normalize(text_segment)
-                        except Exception as exc:
-                            self.log_updated.emit(
-                                (f"Warning: Text normalization failed: {exc}", "orange")
-                            )
-
                         def _qt_check_cancel() -> bool:
                             if self.cancel_requested:
                                 sink_stack.close()
@@ -1049,19 +1041,25 @@ class ConversionThread(QThread):
                             total_characters=self.total_char_count,
                         )
 
-                        run_tts_segment_loop(
-                            text=text_segment,
-                            backend=self.backend,
-                            voice=loaded_voice,
-                            speed=self.speed,
-                            split_pattern=active_split_pattern,
-                            stats=stats,
-                            check_cancel=_qt_check_cancel,
-                            on_progress=_qt_on_progress,
-                            chapter_sink=chapter_sink,
-                            audio_sink=merged_sink if merge_chapters_at_end else None,
-                            on_segment=_qt_on_segment,
-                        )
+                        try:
+                            synthesize_text(
+                                text=text_segment,
+                                tts_context=self._tts_context,
+                                backend=self.backend,
+                                voice=loaded_voice,
+                                speed=self.speed,
+                                stats=stats,
+                                check_cancel=_qt_check_cancel,
+                                on_progress=_qt_on_progress,
+                                chapter_sink=chapter_sink,
+                                audio_sink=merged_sink if merge_chapters_at_end else None,
+                                on_segment=_qt_on_segment,
+                                split_pattern_override=active_split_pattern,
+                            )
+                        except Exception as exc:
+                            self.log_updated.emit(
+                                (f"Warning: TTS failed: {exc}", "orange")
+                            )
 
                         self.processed_char_count = stats.processed_chars
                         current_time = stats.current_time