Agent clients often send message.content as [{type:\"text\",text:\"...\"}]
instead of a plain string. buildNotionMessageStep previously required a
string and silently dropped those turns, so system injects (jailbreak /
agentic conversion) and multimodal user messages never reached Notion.
Normalize string | content-parts | bare string parts via
extractNotionMessageText, and add regression coverage in the transcript
unit suite.
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