When a milestone file's provider identity block omits Tenant and Project
(both are optional since MatchDevelopMilestone fills them from expected),
the bridge's FetchWorkItem call fails with ErrInvalidInput because
planeWorkItemRef requires all three: tenant, project, id.
Fill in missing Tenant/Project from BridgeConfig before building the ref,
mirroring the missing-create path which always uses b.cfg.Tenant/Project.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>