Session permanently corrupted when background Task tool_use is rejected before assistant message is persisted

Bug Description

When a background Task (subagent) is stopped/rejected mid-flight, the assistant message containing the tool_use block is sometimes never written to the session JSONL log, but the corresponding tool_result rejection IS written. This creates an orphaned tool_result that permanently corrupts the session — every subsequent API call fails with:

API Error: 400 "messages.0.content.0: unexpected `tool_use_id` found in `tool_result` blocks: toolu_XXXXX. 
Each `tool_result` block must have a corresponding `tool_use` block in the previous message."

The session becomes unrecoverable. /clear, /mcp, disabling MCP servers — nothing fixes it. Only starting a brand new conversation works.

Steps to Reproduce

1. Start a Claude Code session with a background Task tool call (e.g., a subagent doing a broad codebase search)
2. Stop/reject the background task before it completes (e.g., via the UI cancellation)
3. The session immediately enters a permanent error loop

Root Cause (from JSONL log analysis)

Analyzed the corrupted session log:

• Line 171: Last valid assistant message (uuid 717031c7, a text response)
• Line 172: System message
• Lines 173-174: Queue operations for the background task being killed
• Line 175: tool_result with parent=d8539f11 and tool_use_id=toolu_014bopr75EvmowMNwCsUvaMq (rejection of the stopped task)
• UUID d8539f11 does not exist anywhere in the log — the assistant message containing the tool_use was never persisted
• No context compression events in the session — so the message was not truncated, it was simply never written

The assistant was likely still streaming when the task was stopped. The tool_result rejection was written, but the original tool_use message it references was never committed to the log.

Expected Behavior

Either:

1. The assistant message containing the tool_use should always be persisted before any tool_result referencing it, OR
2. If the tool_use message cannot be persisted, the orphaned tool_result should not be written either, OR
3. On session resume, Claude Code should detect and repair orphaned tool_result blocks (remove them or synthesize a matching tool_use)

Environment

• Claude Code v2.1.47
• Model: Claude Opus 4.6
• macOS Darwin 25.3.0
• The background task was a Task tool call (subagent) for a broad search