fix(mcp): advertise exact output schemas

[jmgilman]

Summary

• advertise explicit inferred outputSchema values for all three MCP tools
• narrow successful collection schemas to non-null arrays and normalize nil runtime slices to []
• document and regress the exact tools/list, StructuredContent, and JSON text-mirror contract

Verification

• mise exec -- go test ./mcpserver -run '^(TestNewRegistersExactlyThreeTools|TestToolsSerializeSuccessfulEmptySlicesAsArrays|TestActualMCPSecureLoop)$' -count=1
• raw stdio MCP initialization, tools/list, empty search_api, and empty-field describe_api smoke passed
• mise exec -- moon run root:check (all eight tasks passed)
• LSP diagnostics on mcpserver/*.go: no issues