#799 Structured output: bare ollama format constraint yields empty replies from cloud thinking models

First live structured run: glm-5:cloud under ollama native format returned EMPTY content (not valid JSON at char 0), and the gateway retry omitted the schema text (for ollama it only rode in the format param). Fixed: (a) ollama adapter also injects the schema instruction into the system message alongside format; (b) the gateway's one corrective retry now embeds schema_instruction() so every provider's model SEES the schema. Verified: structured_output case passes (parsed city/population object).