Fix exceptions raised before task_status.started() losing cause and context

When a task raised an exception before calling task_status.started(),
Nursery.start() unwrapped it from the internal ExceptionGroup using
`raise exc.exceptions[0] from None`, which explicitly set __cause__
to None and __suppress_context__ to True — destroying any cause or
context the original exception carried.

Use raise_saving_context() instead, which preserves both __cause__
and __context__ on the re-raised exception, consistent with how
raise_single_exception_from_group() handles the same pattern.

Fixes #3261.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: worksbyfriday

src/trio/_core/_run.py

@@ -33,7 +33,7 @@
 from .. import _core
 from .._abc import Clock, Instrument
 from .._deprecate import warn_deprecated
-from .._util import NoPublicConstructor, coroutine_or_error, final
+from .._util import NoPublicConstructor, coroutine_or_error, final, raise_saving_context
 from ._asyncgens import AsyncGenerators
 from ._concat_tb import concat_tb
 from ._entry_queue import EntryQueue, TrioToken
@@ -1466,7 +1466,7 @@ async def async_fn(arg1, arg2, *, task_status=trio.TASK_STATUS_IGNORED):
                     # cancel this nursery:
             except BaseExceptionGroup as exc:
                 if len(exc.exceptions) == 1:
-                    raise exc.exceptions[0] from None
+                    raise_saving_context(exc.exceptions[0])
                 raise TrioInternalError(
                     "Internal nursery should not have multiple tasks. This can be "
                     'caused by the user managing to access the "old" nursery in '

src/trio/_core/_tests/test_run.py

@@ -2920,6 +2920,26 @@ async def start_raiser() -> None:
         assert should_be_raiser_exc.exceptions == raiser_exc.exceptions
 
 
+async def test_start_exception_preserves_cause_and_context() -> None:
+    """Regression test for #3261: exceptions raised before task_status.started()
+    should preserve __cause__ and __context__."""
+
+    async def task(*, task_status: _core.TaskStatus[None]) -> None:
+        e = ValueError("foo")
+        e.__cause__ = SyntaxError("bar")
+        e.__context__ = TypeError("baz")
+        raise e
+
+    with pytest.raises(BaseExceptionGroup) as exc_info:
+        async with _core.open_nursery() as nursery:
+            await nursery.start(task)
+    assert len(exc_info.value.exceptions) == 1
+    exc = exc_info.value.exceptions[0]
+    assert isinstance(exc, ValueError)
+    assert isinstance(exc.__cause__, SyntaxError)
+    assert isinstance(exc.__context__, TypeError)
+
+
 async def test_internal_error_old_nursery_multiple_tasks() -> None:
     async def error_func() -> None:
         raise ValueError