go: return failed tool result for empty v2 tool name

[lonegunmanb]

Summary

Fix Go SDK v2 tool.call handling for empty toolName when toolCallId is present.

• Keep payload validation strict for missing sessionId or toolCallId (still returns invalid tool call payload)
• Change empty toolName path to return a structured failed ToolResult instead of JSON-RPC invalid payload error
• Add unit tests for:
  • empty toolName returns failure ToolResult
  • missing toolCallId still returns invalid payload error

This prevents dangling assistant.tool_calls without corresponding tool results in long-running sessions.

Closes #3

Validation

• cd go && go test ./... -run TestClient_HandleToolCallRequestV2 -v