ExceptionGroup.subgroup() returns a fully matching nested subclass group unchanged instead of calling derive() #9576

Summary

subgroup() returns a nested exception group subclass as the same object when all of its leaves match, without calling derive(). split() does call derive(), so eg.split(c)[0] and eg.subgroup(c) disagree. The docs for derive() say: "This method is used by subgroup() and split(), which are used in various contexts to break up an exception group. A subclass needs to override it in order to make subgroup() and split() return instances of the subclass rather than ExceptionGroup."

Reproducer

class MyEG(ExceptionGroup):
    pass
inner = MyEG("inner", [ValueError(1)])
eg = ExceptionGroup("outer", [inner, TypeError(2)])
s = eg.subgroup(ValueError)
print("subgroup:", type(s.exceptions[0]).__name__, s.exceptions[0] is inner)
m, r = eg.split(ValueError)
print("split:   ", type(m.exceptions[0]).__name__, m.exceptions[0] is inner)
print("top:     ", type(inner.subgroup(ValueError)).__name__)

Expected

CPython 3.15.2 (same on 3.12.16):

subgroup: ExceptionGroup False
split:    ExceptionGroup False
top:      ExceptionGroup

Actual

RustPython main (ef023bc):

subgroup: MyEG True
split:    ExceptionGroup False
top:      MyEG

Cause

subgroup() tracks whether anything was dropped and returns zelf when nothing was (if !modified { return Ok(zelf.into()); }), so a group whose leaves all match is never passed to derive() (exception_group.rs#L126-L161). split() right below has no such shortcut. CPython only returns the group itself when the condition matches the group as a whole; otherwise it rebuilds every partially or fully matching nested group with derive(), for split() and subgroup() alike (exceptions.c#L1067-L1100, #L1154-L1157, #L1209-L1233). Dropping the modified shortcut and always calling derive_and_copy_attributes when matching is non-empty would match CPython. PyPy had the same shortcut and just removed it this way (pypy/pypy#5717).

test_exception_group.NestedExceptionGroupSubclassSplitTest.test_split_BaseExceptionGroup_subclass_no_derive_new_override passes because for subgroup() it only checks the leaves.

_{Drafted with help from Claude Code; I reviewed and checked it before posting.}