Add error handling for message conversion in sendChatHistoryMessagesAsync

[Copilot]

Adds try-catch around convertToChatHistoryMessages() in sendChatHistoryMessagesAsync(), matching the error handling pattern used for session.getItems() in sendChatHistoryAsync().

Changes:

• Wrap convertToChatHistoryMessages() call in try-catch block that returns failed OperationResult on error
• Use defensive error coercion: err instanceof Error ? err : new Error(String(err))
• Add test cases for both Error and non-Error thrown values (EH-05, EH-06)

Before:

const chatHistoryMessages = this.convertToChatHistoryMessages(messages);
// Unhandled errors propagate as exceptions

After:

let chatHistoryMessages: ChatHistoryMessage[];
try {
  chatHistoryMessages = this.convertToChatHistoryMessages(messages);
} catch (err: unknown) {
  const error = err instanceof Error ? err : new Error(String(err));
  return OperationResult.failed(new OperationError(error));
}

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