Pair a Review Findings Entry With Its Thread Past the Line Suffix in pr_review.py #3176

The shape

A Copilot round's Review findings: label lists each finding with a FILE:LINE code span:

Review findings:
- [P2, nit, 1 vote] `docs/example.md:12` uses an incorrect noun.
- [P1, moderate, 2 votes] `src/example.py:340` can drop a counted entry.

unthreaded_prose_findings pairs each entry with the round's threads by path. entry_paths keeps the :LINE suffix, so src/example.py:340 never matches a thread whose path is src/example.py, and every entry reads as unthreaded. The digest then reports FINDINGS WITH NO THREAD (N) for every entry, including each that opened a thread, so the reader has to pair them by hand on every round in this shape.

Seen on #3175, where the round listed four entries, three with a thread each, and the digest reported four with no thread.

Expected

An entry's path is read without its trailing :LINE or :START-END, so an entry naming a file a thread of that round is on pairs with it, and only the entry with no thread is counted.