fix: prevent parent task state loss during orchestrator delegation

[hannesrudolph]

Fixes #11172

Summary

Fixes the "parent task disappears in orchestrator mode" bug where parent tasks would rewind to a stale state after a child subtask returned, causing the parent to appear "lost" or "disappeared."

Root Causes

1. JsonStreamStringify lazy mutation bug: saveApiConversationHistory() and saveClineMessages() passed live array references to safeWriteJson(), which uses JsonStreamStringify that lazily reads elements during streaming. Elements pushed after construction were silently dropped.

2. Silent error swallowing: Both save functions swallowed all write errors, so flushPendingToolResultsToHistory() cleared in-memory state even when the disk write failed. The parent was then destroyed from the stack, leaving only stale disk state.

3. Destructive getTaskWithId(): When api_conversation_history.json was missing, it permanently deleted the task's HistoryItem from globalState — turning transient filesystem errors into permanent data loss.

4. Unprotected read paths: readTaskMessages() had no try/catch around JSON.parse; readApiMessages() caught parse errors but re-threw them.

Changes

• src/core/task/Task.ts: Snapshot arrays with structuredClone() before save for deep isolation from JsonStreamStringify's lazy reads; return boolean from save methods; guard flushPendingToolResultsToHistory() to retain in-memory state on failure; add retrySaveApiConversationHistory() with exponential backoff (3 attempts: 100ms, 500ms, 1500ms)
• src/core/webview/ClineProvider.ts: Make getTaskWithId() non-destructive (return empty history instead of deleting task); add try/catch around JSON.parse in getTaskWithId(); add flush retry with user-visible warning in delegateParentAndOpenChild()
• src/core/task-persistence/taskMessages.ts: Add try/catch around JSON.parse in readTaskMessages()
• src/core/task-persistence/apiMessages.ts: Return [] instead of re-throwing in readApiMessages() catch blocks

Tests

• New and updated tests across 5 test files covering all fix scenarios
• Full test suite passes (5,470 tests across 371 files, 0 regressions)

[roomote]

Rooviewer   See task

Reviewed changes in a6537e6 (rebased). No new issues found. The PR-specific fixes (structuredClone snapshots, boolean save returns, guarded flushPendingToolResultsToHistory, non-destructive getTaskWithId, delegation retry with user warning, graceful JSON.parse handling) remain correct and well-tested. Rebased changes from main (Bedrock AI SDK refactor, baseten provider, disabledTools, dependency updates) are orthogonal and introduce no conflicts.

• Snapshot arrays before safeWriteJson to prevent JsonStreamStringify lazy-read mutation
• Boolean return from save methods with guarded flushPendingToolResultsToHistory
• Non-destructive getTaskWithId (no longer deletes task on missing file)
• Graceful JSON.parse error handling in readTaskMessages / readApiMessages
• Retry logic in delegateParentAndOpenChild with CRITICAL logging
• 13 new tests covering all fix scenarios

Previous reviews

• d54e340: Review #1
• 2491615: Review #2

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