fix: walk a group's __cause__/__context__ in the exception node walker

Review finding: _exc_children returned only .exceptions for a group, so
_is_session_expired_error missed a session-expiry marker (or the
InterruptedError override) hanging off a group's __cause__/__context__
that main used to inspect. Groups now yield nested + chain like every
other node; _flatten_messages' "group str() is opaque" rule is unchanged.

tests/tools/test_mcp_tool_session_expired.py

@@ -66,6 +66,20 @@ def test_is_session_expired_traversal_is_budget_bounded():
     assert _is_session_expired_error(exc) is False
 
 
+def test_is_session_expired_walks_group_chain():
+    """A group's own ``__cause__``/``__context__`` are inspected like any node's: a marker there classifies
+    as expired, and an InterruptedError there still overrides a marker inside the group."""
+    from tools.mcp_tool_errors import _is_session_expired_error
+
+    group = ExceptionGroup("task group", [ValueError("unrelated")])
+    group.__context__ = RuntimeError("session terminated")
+    assert _is_session_expired_error(group) is True
+
+    group = ExceptionGroup("task group", [RuntimeError("session terminated")])
+    group.__context__ = InterruptedError()
+    assert _is_session_expired_error(group) is False
+
+
 # ---------------------------------------------------------------------------
 # Handler integration — verify the recovery plumbing wires end-to-end
 # ---------------------------------------------------------------------------

tools/mcp_tool_errors.py

@@ -305,9 +305,11 @@ _EXC_TRAVERSAL_MAX_NODES = 10_000
 
 
 def _exc_children(exc: BaseException) -> List[BaseException]:
-    """Sub-exceptions of a group, else ``__cause__``/``__context__`` when they are exceptions."""
-    nested = getattr(exc, "exceptions", None)
-    return list(nested) if nested else [c for c in (exc.__cause__, exc.__context__) if isinstance(c, BaseException)]
+    """A group's sub-exceptions (if any) followed by ``__cause__``/``__context__`` when they are exceptions — a
+    group raised inside an ``except`` block carries the caught error as ``__context__``, so the chain is never
+    skipped."""
+    nested = getattr(exc, "exceptions", None) or ()
+    return [*nested, *(c for c in (exc.__cause__, exc.__context__) if isinstance(c, BaseException))]
 
 
 def _iter_exception_nodes(exc: BaseException) -> List[BaseException]: