`deque.index` result changed in 3.14.7 when a bound's `__index__` mutates the deque

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(written by claude)

Bug description

gh-150750 (merged in 3.14.7 via #150921) changed the result of deque.index when a bound's __index__ mutates the deque.
The change is not mentioned in the NEWS entry, which only describes the free-threading race, and is not covered by a test.

from collections import deque

d = deque([1, 2, 3])

class Grow:
    def __index__(self):
        d.extend([2, 2, 2])
        return -1

print(d.index(2, Grow()))

3.14.6 and earlier raise ValueError: deque.index(x): x not in deque.
3.14.7 prints 5.

Before #150779, stop defaulted to Py_SIZE(deque) read during argument parsing, before the start converter called __index__.
In the example, stop was captured as 3, start=-1 then resolved against the live size 6 to give 5, and start > stop clamped the search to an empty window.
After #150779, deque_index_impl reads the size once, after argument conversion, so both bounds resolve against the mutated size and the search finds the element.

The new behavior is self-consistent (one size snapshot, taken after both converters run) and matches what list.index does, so this is probably fine to keep.
Reporting it because the change is user-visible in a patch release: it may deserve a changelog note and a regression test, or reverting if the old behavior was intended.

Your environment

• CPython versions tested on: 3.14.0, 3.14.2, 3.14.4 (old behavior); 3.14.7, 3.15.0rc1 (new behavior)
• Operating system and architecture: macOS arm64

Linked PRs

• gh-157784

Bắt đầu bằng cách tái hiện ví dụ deque.index và xem xét phần triển khai deque.index, mục NEWS và PR #157784 được liên kết. Hoàn tất có nghĩa là xác định liệu hành vi của 3.14.7 có nên được giữ lại hay không và, nếu có, ghi lại thay đổi तथा bổ sung phần kiểm thử hồi quy còn thiếu.