fix(anthropic): continue after emulated web search

[SimianW]

Summary

• continue Kiro generation after an intercepted Anthropic Path B WebSearch instead of ending with a raw search dump
• preserve Anthropic server_tool_use and web_search_tool_result blocks in the original SSE message before streaming the model's natural answer
• retain server-search history across mixed client-tool turns so the model can continue the original task
• bound internal search continuation to five rounds and remove web_search on the final continuation request
• fall back to the deterministic search summary if model continuation fails
• accept Anthropic server WebSearch blocks when clients replay conversation history

Testing

• pytest -q (1701 passed)
• live Anthropic SSE verification produced server_tool_use, web_search_tool_result, then natural text, with stop_reason=end_turn

Closes #258