[Bug]: xAI grok-4.3 drops optional multiline string args from MCP tool calls — AgentMail sends blank emails (docs recommend the affected combo)

#58345 Workaround P2 comp/agent

[Bug]: xAI grok-4.3 drops optional multiline string args from MCP tool calls — AgentMail sends blank emails (docs recommend the affected combo)

On xai-oauth with grok-4.3, optional multiline string args are dropped from MCP tool calls, causing AgentMail to send blank emails while reporting success.