The transport half of the effort_fallback wiring got a test last round; the
assembly half did not. Mutating _guard_thinking's fallback=source.effort_fallback
to a hardcoded "error" leaves the whole suite green, yet a zhipu/glm-5.3 source
carrying REASONING_EFFORT=medium + EFFORT_FALLBACK=nearest goes from assembling
fine to being refused at assembly. Pin it down: from_env must return a client.
iomgaa
bb9ef038c7