Collect solutions from a Conflict Solution PR merge when squashing

[plebioda]

squash_to_merge builds the squashed merge commit's combined note from _collect_commits_for_squash, which walked the first-parent chain only. When a merge's conflicts are resolved via a solution->conflict PR, the resolution/fix commits (which carry the solutions) live on the second parent of that PR merge commit, so the first-parent walk never saw them. The squashed note ended up with conflict_context but no solutions, so mergai rebase later found a recorded conflict with no solution to reapply -- forcing a manual re-resolve of an already-solved conflict.

This was latent for a long time (resolutions used to land as inline, single-parent commits on the first-parent chain) and only started corrupting notes once conflicts began being resolved via a 2-parent solution-PR merge.

Expand a solution/review PR merge (a merge whose second parent is not the upstream merge commit) into the commits it brought in, so their notes -- and their solutions -- are collected. The mergai merge commit itself (second parent == the upstream merge commit) is still kept as a leaf and never descended into, so upstream history is not pulled in. This mirrors _collect_squashed_commit_entries, keeping the combined note's solutions consistent with the squashed_commits record.

Add a regression test that builds the solution-PR-merge topology and asserts the solutions are collected and combined.

[Copilot]

Pull request overview

Fixes squash_to_merge note construction so conflict-resolution “solutions” are not lost when conflicts were resolved via a dedicated Conflict Solution PR that lands as a 2-parent PR-merge commit (solutions on the second-parent side).

Changes:

• Expand solution/review PR-merge commits during _collect_commits_for_squash so second-parent-side resolution/fix commits (and their notes/solutions) are collected.
• Update _collect_commits_for_squash documentation to describe the expanded traversal behavior and its rationale.
• Add a regression test that constructs the solution-PR-merge topology and asserts solutions are collected into the combined note.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/mergai/app.py	Enhances squash commit collection to descend into second-parent-side commits for solution/review PR merges, preventing solution notes from being dropped.
tests/test_squash_collects_pr_solutions.py	Adds regression coverage for collecting solutions across a Conflict Solution PR merge topology.

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