fix(gitea-mq): match the required-check fallback to the authoritative ruleset

[cameronraysmith]

Ruleset 16212553 requires nixbot/nix-eval, nixbot/nix-build and
nixbot/effects; the module's fallback listed only the first two. The fallback
does not fire today, because ResolveRequiredChecks prefers a non-empty
forge-derived list, but a fallback weaker than the forge is worse than none:
it would silently reduce the gate exactly when the forge reports nothing.

The downgrade path is narrower than a missing ruleset. GetRequiredChecks
errors propagate rather than falling back, so an unreadable ruleset or lost
read access stalls the queue instead of weakening it. The fallback fires only
on a successful empty list — a ruleset deleted, emptied, retargeted off the
default branch, or reduced to gitea-mq-owned contexts, which GetRequiredChecks
subtracts.

Second coupling, recorded because it is the one that bites: nixbot/effects is
posted only when at least one effect runs. effects_run.py returns early on an
empty effect set, so effects_started is never called and the context is never
posted. A required check that is never posted blocks a PR rather than failing
it, so requiring it is safe only while the default branch keeps
effects_on_pull_requests true with a non-empty effect set. nixbot.toml and
this list must move together.

[cameronraysmith]

This pull request is part of a Mergify stack:

#	Pull Request	Link
1	feat(gitea-mq): raise the batch cap to twenty for flake-update waves	#3014
2	fix(gitea-mq): match the required-check fallback to the authoritative ruleset	#3015	👈