feat: enhance activity and task APIs with improved payload handling and response models

app/api/v1/activities/models.py

@@ -1,11 +1,18 @@
 """Pydantic schemas for activity endpoints."""
 from __future__ import annotations
 
-from typing import Any, Literal
+from typing import Literal
 
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
+
+
+class ActivityCommentBody(BaseModel):
+    text: str = Field(..., min_length=1, max_length=2000)
 
 
 class ActivityCommentPayload(BaseModel):
-    type: Literal["comment"]
-    payload: dict[str, Any]
+    type: Literal["comment"] = "comment"
+    payload: ActivityCommentBody
+
+    def extract_text(self) -> str:
+        return self.payload.text.strip()