test: raise coverage on the least-covered modules

[BryanFRD]

Sonar reports 67.8% line coverage on main (3360 of 10449 lines uncovered). Most of the gap sits in a few areas: forge clients (github, gitlab, gitea), version_diff and api_check, the why/graph/preview renderers, config migrate, doctor checks, rollback and the release execution paths (floating tags, release commit, finalize).

Add tests that guard real behaviour in those areas: HTTP clients against a local fake server, git paths against temporary repositories, renderers and migrations on their edge cases. No assertion-free or tautological tests.

[BryanFRD]

Coverage on main went from 67.8% to 80.2% (uncovered lines 3360 to 2112) with #1222, #1223, #1224 and #1225. Fixes found along the way shipped in #1226, #1229 and #1231; remaining findings are tracked in #1232 to #1235.