Fix: Validate tool name presence in MCP tools/call requests

[Copilot]

The TestHandleRequest_ToolsCall_InvalidJSON test was failing because requests with missing or empty tool names returned "unknown tool: " instead of "invalid params".

Root Cause

Valid JSON structures like {"arguments": {}}, null, or {} successfully unmarshal but leave callParams.Name as an empty string. The code then called callTool("") which returned "unknown tool: " rather than validating the parameter first.

Changes

internal/sys/sys.go

• Added validation after unmarshaling to check if callParams.Name is empty
• Returns "invalid params: missing tool name" before attempting tool lookup

internal/sys/sys_test.go

• Removed "empty tool name" case from TestHandleRequest_ToolsCall_UnknownTool since explicit empty strings are now correctly categorized as invalid params rather than unknown tools

// After unmarshal, validate tool name is present
if callParams.Name == "" {
    log.Printf("Tool call missing name field")
    return nil, fmt.Errorf("invalid params: missing tool name")
}

This distinguishes between malformed requests (missing required field) and valid requests for non-existent tools, providing clearer error semantics.

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