fix: return isError for argument validation failures

[blackwell-systems]

Fixes #1952.

Problem

When tool argument unmarshalling fails (wrong types, malformed JSON), the handler returns (nil, err) which the SDK converts to a JSON-RPC protocol error (-32603). Agents cannot see protocol errors in their context window, so they cannot self-correct.

Fix

Return isError: true with the validation error message instead of a Go error. Two sites in server_tool.go:

• NewServerTool (line 135)
• NewServerToolWithContextHandler (line 159)

Before:

if err := json.Unmarshal(req.Params.Arguments, &arguments); err != nil {
    return nil, err // protocol error: agent sees nothing
}

After:

if err := json.Unmarshal(req.Params.Arguments, &arguments); err != nil {
    return &mcp.CallToolResult{
        Content: []mcp.Content{&mcp.TextContent{Text: fmt.Sprintf("invalid arguments: %s", err)}},
        IsError: true,
    }, nil // tool execution error: agent sees the message and can retry
}

Tests

• TestNewServerTool_InvalidArguments_ReturnsIsError
• TestNewServerToolWithContextHandler_InvalidArguments_ReturnsIsError
• TestNewServerTool_ValidArguments_Succeeds