feat: prevent successive assistant messages in agent loop

[MohamedAkbarally]

Prevent Successive Assistant Messages in Agent Loop

Summary

Add automatic user message injection to prevent successive assistant messages when the agent responds without tool calls

Details

• Agent loop now checks if assistant message has no tool calls and no user messages
• Automatically injects UserMessage(content="Please continue the task") to maintain valid message alternation
• Skips injection if next turn will show "turns remaining" warning to avoid redundant prompts
• Prevents API errors from LLM providers that require strict user/assistant message alternation

Test Coverage

• Added test_no_successive_assistant_messages to verify automatic continuation message injection
• Test validates message is added only when needed (not when turns remaining warning will appear)

[declanjackson]

lgtm!