Threading inconsistency

When looking at https://lists.apache.org/list?dev@pekko.apache.org:2025-12 , opening `[VOTE] Release Apache Pekko (Core) 2.0.0-M1-RC1` shows a thread of 5 messages.

However, when looking at individual messages, like https://lists.apache.org/thread/w98n335q42zvv1fmgt1p8x0sx5523typ, they do not find https://lists.apache.org/thread/jrzh887gvlh9ttgwtvsky1f88m65xk5m as their parent, and vice-versa.

I have no preference between those two interpretations (I haven't looked at the details but GMail also considers this thread broken), but it'd be good if it were consistent ;)

Possibly related to #244

Start by comparing the list view and individual message views linked in the report, then read the related issue #244 for context. The fix is complete when parent and child relationships are represented consistently between the thread summary and individual message pages.

Written by the indexing model from the issue text.