Refactor agent_runner: extract methods from send() and _build_options()

[nikitaxru]

Summary

• Break down AgentSession.send() (~100 lines → ~40) into 4 focused methods: _process_assistant_message(), _apply_result_message(), _read_tool_state(), _build_success_result()
• Break down AgentRunner._build_options() (~100 lines → ~58) by extracting _build_mcp_servers(), _build_allowed_tools(), and moving hardcoded tool lists into Final[tuple] constants
• Add 5 characterization tests covering previously untested behavioral contracts (mailbox/comm tools, ToolState consumption/reset, AGENT_OUTPUT events)

Test plan

• All 66 test_agent_runner.py tests pass (61 existing + 5 new)
• Full test suite: 2002 passed
• ruff check — clean
• ruff format --check — clean
• mypy — no issues
• Pre-commit hooks pass

🤖 Generated with Claude Code

[cursor]

PR Summary

Low Risk
Primarily a refactor of agent session option-building and response-processing code with added tests; behavior should be unchanged, but regressions could affect tool allowlists, event emission, or token/cost reporting.

Overview
Refactors AgentSession.send() by extracting focused helpers for processing AssistantMessage text/rate-limit detection, applying ResultMessage cost/token/session updates, consuming/resetting ToolState side-channel flags, and assembling the final AgentResult.

Refactors AgentRunner._build_options() by splitting MCP server construction vs allowed-tool list assembly, moving hardcoded tool names into Final constants, and adding a lazy import path when mailbox enables the comm MCP server.

Adds characterization tests to lock in behavior around mailbox enabling comm tools, ToolState consumption/reset semantics, and publishing AGENT_OUTPUT events per TextBlock.

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