fix: tolerate missing Compose coverage on legacy branches

[aptend]

What this PR does / why we need it

Legacy MatrixOne branches such as 4.2-dev do not mount /coverage or set GOCOVERDIR in their Compose configuration. Their Compose BVT can therefore pass while the reusable workflow fails afterward when it requires a coverage profile.

This change:

• keeps the successful Compose BVT result when the checked-out legacy Compose configuration has no coverage wiring;
• still requires both BVT coverage profiles on main;
• lets legacy branches merge UT plus standalone BVT coverage when only Compose coverage is unsupported;
• retains both BVT manifests, so complementary group and generation validation is unchanged.

Observed in MatrixOne run: https://github.com/matrixorigin/matrixone/actions/runs/30250338336

Testing

• git diff --check
• exercised main/legacy profile-count branches and Compose capability detection with shell fixtures
• verified main still rejects a missing Compose profile