✨(agents) add Sentry instrumentation for agents

Add Sentry observability to the `agents`. Introduce a dedicated
`observability.py` module. Refactor both agents to use the shared
observability layer and extract task helpers into a new `tasks.py` module,
making task execution easier to instrument and maintain.

src/agents/tasks.py

@@ -0,0 +1,42 @@
+"""Helpers for managing asyncio tasks."""
+
+import asyncio
+import logging
+from collections.abc import Callable
+from typing import Any
+
+
+def done_callback(
+    logger: logging.Logger,
+    tasks: set[asyncio.Task],
+    description: str,
+    *,
+    on_success: Callable[[Any], None] | None = None,
+) -> Callable[[asyncio.Task], None]:
+    """Build a done-callback for a background task.
+
+    Meant to be passed to `asyncio.Task.add_done_callback`.
+
+    Args:
+        logger: Logger used to report failures, so records keep the caller's
+            logger name.
+        tasks: Set the task was registered in; the task is discarded from it.
+        description: Human-readable intended action
+        on_success: Optional callback invoked with the task's result when it
+            completes without error.
+
+    Returns:
+        A callback suitable for ``task.add_done_callback(...)``.
+    """
+
+    def _finalize(task: asyncio.Task) -> None:
+        tasks.discard(task)
+        if task.cancelled():
+            return
+        if (exc := task.exception()) is not None:
+            logger.exception("failed to %s", description, exc_info=exc)
+            return
+        if on_success is not None:
+            on_success(task.result())
+
+    return _finalize