Handle empty Go tool names as failed tool results

[slegarraga]

Fixes #1540.

Summary

• return a structured failed pending-tool-call result when the Go broadcast path receives an empty tool name with a valid tool call id
• keep the missing-tool-call-id case unchanged, since a provider-visible tool result cannot be associated without an id
• add a regression test that verifies the JSON-RPC session.tools.handlePendingToolCall payload

Validation

• gofmt -w go/session.go go/session_test.go
• go test ./... -run TestSession_HandleBroadcastEventRespondsToMissingToolName -count=1
• go test $(go list ./... | grep -v '/internal/e2e')\n\nNot run locally:\n- golangci-lint run ./... (golangci-lint is not installed in this environment)\n- full go test ./... (the internal/e2e package run did not complete locally without the shared E2E setup)