fix(roadmap): alternatives posts a comment only when its verdict changes #4054

Problem

apply_alternatives posts a comment and a block for every check, also when the verdict and the tool are the same as in the last check. A recheck every 30 days (DEFAULT_ROADMAP_ALTERNATIVES_RECHECK_DAYS) adds a comment with no news. The comment thread of a long-lived item fills with the same text.

Expected

In apply_alternatives, compare the new verdict and tool with the recorded alternatives.verdict and alternatives.tool.

• Write the comment only when one of them changed.
• Update the block and the marks (the date) in both cases.

The order rule of #2951 does not change. A recheck with the same result still updates alternatives.checked_at, so the item is not due again before the recheck days pass.

Acceptance criteria

• uv run pytest tests/roadmap/test_roadmap_alternatives_recheck.py -k same_verdict - a recheck with the same verdict and tool posts no second comment. It updates the date.
• uv run pytest tests/roadmap/test_roadmap_alternatives_recheck.py -k changed_verdict - a recheck with a new verdict, or with a new tool, posts one comment.
• uv run pytest tests/roadmap/test_roadmap_alternatives_recheck.py -k first_check - a first check, with no recorded verdict, posts its comment.

Every command above fails on the unchanged code. The pull request cites the fail-to-pass run (devops ci fail-to-pass --base <ref>).

Feasibility

I read alternatives.py (apply_alternatives, _is_due, AlternativesRecordKey) at origin/main. The recorded keys alternatives.verdict and alternatives.tool are written in apply_alternatives. I did not run the code.

Existing tool considered

The item record marks that exist. No library is needed.

Cost

Build: about 15k tokens and 1 h. Keep: about 1k tokens per quarter. Fix: about 5k tokens and 0.3 h.

Related

Part of #3027. #2951. Order: after the issue #4049 (both change alternatives.py).