Summary

• Fix HTTP 400 errors caused by orphaned tool_result blocks in the conversation history sent to the Anthropic API.
• After truncating the conversation to ctx_size, strip any leading role="tool" messages whose corresponding assistant (tool_use) message was cut off by the truncation boundary.

Closes #415

Root Cause

Each tool_call DB record expands into 2 messages (assistant + tool) during history reconstruction, so conversation[-ctx_size:] can split a pair — keeping the tool result but dropping the tool use. The Anthropic API requires every tool_result to have a matching tool_use in the preceding assistant message.

Test plan

• Reproduce with a session containing ~50 tool-call rounds and context_window_size = 100
• Verify the LLM call succeeds after the fix
• Verify normal sessions without tool calls are unaffected