fix: filter out empty parts in Gemini format conversion to prevent API errors

[roomote]

This PR attempts to address Issue #9816.

Problem

The Gemini API was receiving malformed requests with empty data fields, causing the error:

GenerateContentRequest.contents[4].parts[2].data: required oneof field 'data' must have one initialized field

Root Cause

The convertAnthropicContentToGemini function uses flatMap which can return empty arrays when:

• thoughtSignature blocks are excluded
• tool_result blocks have no content
• Unsupported content block types are encountered

These empty arrays were being included in the parts array sent to Gemini, causing API errors.

Solution

1. Added filtering logic to remove empty parts from the flatMap results
2. Enhanced convertAnthropicMessageToGemini to properly handle empty parts arrays
3. Added comprehensive test coverage for edge cases

Testing

• All existing tests pass
• Added 6 new test cases covering various edge cases with empty content blocks
• Verified the fix prevents sending empty parts to the Gemini API

Feedback and guidance are welcome!

[roomote]

Rooviewer   See task on Roo Cloud

Review complete. Found 1 code quality observation:

• The filter added at lines 139-145 is redundant since flatMap already handles empty arrays

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}

[roomote]

This filter is redundant because flatMap already handles empty arrays by flattening them away (an empty array [] contributes nothing to the flattened result). When the callback returns [] on lines 60, 84, 106, or 135, flatMap automatically excludes those from the final array. The filter won't actually filter anything since flatMap has already removed empty arrays, and every Part object returned in the code has at least one property (making Object.keys(part).length > 0 always true for valid Parts).

_{Fix it with Roo Code or mention @roomote and request a fix.}