Handle delegation when pending tool-result flush and retry save both fail #2188

Summary

Handle the delegation failure path where flushPendingToolResultsToHistory() returns false and retrySaveApiConversationHistory() also fails. Delegation currently treats the failure as non-fatal and continues. When the parent resumes, its history can end with an assistant tool_use without the required tool_result, which can cause an Anthropic 400 error.

Requested by @PierrunoYT as a separate follow-up to #2113.

Failure path

1. A parent task attempts to delegate through delegateParentAndOpenChild.
2. flushPendingToolResultsToHistory() fails. An assistant-persistence failure can cause it to return before adding pending results to history.
3. The retry save also fails. The caller warns and continues delegation.
4. The parent later enters resumeAfterDelegation() with no new user content.
5. The outgoing history can contain a trailing assistant tool_use without its tool_result.

Saving existing history is not equivalent to flushing pending tool results.

Required changes

• Define safe delegation behavior when both the flush and retry save fail.
• Define the user-visible failure or recovery behavior, including whether the parent stays open and how delegation can be retried.
• Ensure the parent cannot send an unmatched trailing tool_use after this failure.
• Preserve pending results for recovery and avoid duplicate tool results or duplicate child tasks during retries.
• Keep the generic send-time normalizer's handling of trailing tool calls unchanged: a trailing tool call can still have a result pending.

This issue does not prescribe throwing immediately after a transient write error. The delegation behavior needs a separate decision and dedicated tests.

Affected areas

• src/core/webview/ClineProvider.ts: delegateParentAndOpenChild and delegation recovery.
• src/core/task/Task.ts: flushPendingToolResultsToHistory, retrySaveApiConversationHistory, and resumeAfterDelegation.
• Delegation specifications and persistence-failure tests.

Acceptance criteria

• A regression test covers a failed flush followed by a failed retry save, including failure before pending results enter history.
• The selected behavior prevents an API request with an unmatched trailing assistant tool_use.
• Tests cover recovery or retry without lost results, duplicate results, or duplicate child tasks.
• Tests confirm that successful delegation remains unchanged.
• The user-visible failure and recovery behavior is documented.

Context

This is an existing delegation failure path, not a regression introduced by #2113. That PR repairs malformed outgoing history without changing delegation policy.

• PR: [Fix] Anthropic tool_use / tool_result pairing errors (400) #2113
• Review thread: [Fix] Anthropic tool_use / tool_result pairing errors (400) #2113 (comment)