refactor(ai-client): unify OpenAI-compatible path to AI SDK generateText

Eliminate the dual code path (raw fetch vs AI SDK) for text and vision.
All providers now go through createLanguageModel() + generateText(),
removing chatOpenAiCompatible/analyzeOpenAiCompatible, the manual Usage
type, summarizeUsage, and responseFormat plumbing from 8 call sites.

Key fix: @ai-sdk/openai v3 defaults to the Responses API (/responses);
DeepSeek only supports Chat Completions, so we use .chat() explicitly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/engine/agents/characterDesigner.ts

@@ -56,7 +56,7 @@ async function runDesignLLM(
         content: buildCharacterDesignerUserMessage(charName, session),
       },
     ],
-    { temperature: 0.7, responseFormat: "json_object", tag: "character-designer" },
+    { temperature: 0.7, tag: "character-designer" },
   );
   return parseJsonLoose<CharacterDesignOutput>(raw);
 }