[OpenHands SDK] Validation errors split a single tool-call response in conversation history

## Observed symptom [Bug]: Validation errors split a single tool-call response in SDK conversation history ### Is there an existing issue for the same bug? - [x] I have searched existing issues and this is not a duplicate. ### Bug Description Validation errors can split one LLM tool-call response into multiple assistant messages in the SDK's common conversation history, before it is serialized for a provider. For a response containing A (valid), B (missing a required argument), and C (valid), the error event for B interrupts the action-event batch. The SDK reconstructs two assistant messages instead of the original single response, with A's result appearing after the message containing C. This is an SDK history-construction defect. It does not require Anthropic-specific conversion or concurr ## Literal error text ``` Validation errors split tool-call response ``` ## Environment GitHub issue OpenHands/software-agent-sdk #5354 ## Trigger / context ### Is there an existing issue for the same bug? - [x] I have searched existing issues and this is not a duplicate. ### Bug Description Validation errors can split one LLM tool-call response into multiple assistant messages in the SDK's common conversation history, before it is serialized for a provider. For a response containing A (valid), B (missing a required argument), and C (valid), the error event for B interrupts the action-event batch. The SDK reconstructs two assistant messages instead of the original single response, with A's result appearing after the message containing C. This is an SDK history-construction defect. It does not require Anthropic-specific conversion or concurrent tool execution: the reproduction uses `tool_concurrency_limit=1`. The downstream symptoms depend on the target format and provider; this report does not claim that every API rejects the history or loses results. ### Expected Behavior At the common SDK history layer, keep all tool calls from one response together, in their original order and with their original IDs, followed by exactly one actual result or validation error for each call. Invalid calls must not execute. Preserve those calls ## Source evidence - https://github.com/OpenHands/software-agent-sdk/issues/5354 (2026-09-28T07:19:11Z) — #5354 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:46:06.809152Z - scout_id: Cf55a5ab6e2 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome.