fix: use placeholder for empty tool result content to fix Gemini API validation

[daniel-lxs]

Summary

Fixes Gemini API validation error where empty tool result content causes "Unable to submit request because it must include at least one parts field" error.

Problem

Users were encountering an ApiProviderError when using Gemini models via OpenRouter because tool result messages with empty content fail Gemini's validation requirements.

Root Cause

While most tool execution paths add fallback content, empty content can still reach the API from:

• Legacy/persisted conversations stored before fallbacks were added
• Resumed tasks from history with empty tool results
• Context condensation where tool results may lose content
• MCP tools with empty responses (processToolContent returns "" for empty results)

Solution

Applied a defensive fix at the transformation layer in src/api/transform/openai-format.ts:

content: content || "(empty)",

This is the correct location because:

• It's the last transformation before the API call
• It catches ALL edge cases regardless of origin
• It doesn't require changes across multiple tool execution paths

Testing

• Added 3 test cases covering empty string, undefined, and empty array content
• All 27 tests in openai-format.spec.ts pass

PostHog Reference

https://us.posthog.com/error_tracking/019bb191-73dd-7471-8079-4723c6b2efa0

Closes ROO-517

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Important

Fixes Gemini API validation by using "(empty)" placeholder for empty tool result content in convertToOpenAiMessages() in openai-format.ts.

• Behavior:
  • Fixes Gemini API validation error by using "(empty)" placeholder for empty tool result content in convertToOpenAiMessages() in openai-format.ts.
  • Handles cases with empty string, undefined, and empty array content.
• Testing:
  • Adds test cases for empty content scenarios in openai-format.spec.ts.
  • Ensures all 27 tests in openai-format.spec.ts pass.
• Misc:
  • Closes issue ROO-517.

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[roomote]

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Review complete. No issues found.

This is a well-targeted fix that correctly handles empty tool result content for Gemini API compatibility. The change is minimal, properly placed at the transformation layer, and has good test coverage.

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