fix: HITL session persistence, tool events, and breakpoint interaction

[cristipufu]

Summary

Commit 1: Persist session after HITL resume and emit tool completed events

• After HITL resume, checkpoint restore creates separate session copies per executor. Extract the most complete session (highest message count) and persist it to KV storage so the next turn has valid history.
• Handle AgentExecutorResponse wrapper in _extract_tool_state_events and _extract_contents so function_result from executor_completed data is properly found.
• Emit ToolCallEnd in close_message() for pending tool calls interrupted by HITL suspension (clears stale _pending_tool_calls state).
• Track pending tool nodes (STARTED without COMPLETED) across stream iterations and synthesize COMPLETED events on HITL resume.

Commit 2: Prevent breakpoint+HITL infinite loop and duplicate events

• Load breakpoint skip counts on HITL resume so the executor passes through without re-firing the breakpoint (prevents the infinite breakpoint -> HITL -> breakpoint loop on same node).
• Clear _last_checkpoint_id after loading breakpoint state to prevent the next fresh turn from being mistaken for a breakpoint resume.
• Detect stale checkpoints from previous turns by capturing a baseline before workflow.run() and comparing after breakpoint fires.
• Fall back to _resumed_from_checkpoint_id (instead of None) when no new checkpoint was created, preventing replay from scratch that caused duplicate handoff_to_billing_agent events (4x).

Test plan

• test_no_duplicate_tool_calls_on_breakpoint_resume — breakpoints=* + HITL: exactly 1 handoff_to_billing_agent (was 4x before fix)
• test_second_turn_after_breakpoint_and_hitl — multi-turn stale checkpoint detection still works
• test_breakpoint_then_hitl_does_not_loop — breakpoint + HITL on same node completes without infinite loop
• test_breakpoint_on_all_nodes_with_hitl — breakpoints=* + HITL completes successfully
• test_tool_node_completed_after_hitl_resume — tool nodes emit both STARTED and COMPLETED across HITL
• All 12 HITL E2E tests pass
• All 15 streaming tests pass

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