fix: add pre-send validation for tool_use/tool_result pairing

[roomote-v0]

Related GitHub Issue

Closes: #11777

Description

This PR attempts to address Issue #11777 by adding a pre-send validation safety net that ensures every tool_use block has a corresponding tool_result block before API requests are sent to Anthropic.

The codebase already has validateAndFixToolResultIds() which validates at message insert time. However, several post-processing steps run between insert-time and the actual API call (getEffectiveApiHistory, mergeConsecutiveApiMessages, buildCleanConversationHistory) that can introduce new mismatches.

Changes:

1. New function validateMessageHistoryBeforeSend() in src/core/task/validateToolResultIds.ts:

   • Iterates through the final message array looking for assistant messages with tool_use blocks
   • Checks the following user message has matching tool_result blocks for each tool_use
   • Injects placeholder tool_result blocks for any missing pairings
   • Inserts synthetic user messages when none follow an assistant message with tool_use
   • Reports mismatches to telemetry via MissingToolResultError
   • Returns the original array reference (no allocation) when no fixes are needed

2. Integration in Task.ts: Called right after buildCleanConversationHistory() and before createMessage() -- the last possible moment before the API call.

Feedback and guidance are welcome.

Test Procedure

• 7 new unit tests added to src/core/task/__tests__/validateToolResultIds.spec.ts covering:
  • All tool_use blocks have matching tool_results (no modification)
  • Missing tool_results for multiple tool_use IDs (placeholders injected)
  • No following user message (synthetic message inserted)
  • Multiple assistant messages with tool_use blocks in conversation
  • Messages without tool_use blocks (passthrough)
  • Assistant messages with string content (passthrough)
  • Next message being assistant instead of user (synthetic user message inserted)
• All 34 tests pass: cd src && npx vitest run core/task/__tests__/validateToolResultIds.spec.ts
• Lint and type checks pass across all packages

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes.
• Documentation Impact: No documentation updates needed (internal validation logic).
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

No documentation updates required. This is an internal validation mechanism.

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