✨(summary) relaxed WhisperX response model

When going through AlbertAPI, timestamp are not provided at the word level.
This adds default values so that the summary external contract stays the same,
while giving us compatibility wisht AlbertAPI.

src/summary/summary/core/shared_models.py

@@ -40,7 +40,9 @@ class Segment(BaseModel):
         title="Segment Text", description="Transcribed text for the segment."
     )
     words: tuple[WordSegment, ...] | None = Field(
-        title="Word Segments", description="List of word segments within the segment."
+        title="Word Segments",
+        description="List of word segments within the segment.",
+        default=None,
     )
     speaker: str | None = Field(
         default=None, title="Speaker", description="Speaker identifier for the segment."
@@ -54,7 +56,9 @@ class WhisperXResponse(BaseModel):
         title="Segments", description="List of transcribed segments."
     )
     word_segments: tuple[WordSegment, ...] = Field(
-        title="Word Segments", description="List of word segments."
+        title="Word Segments",
+        description="List of word segments.",
+        default_factory=tuple,
     )