Fix parallel image tool_result handling in Claude mapper

[xiaodream551-a11y]

Summary

• keep all functionResponse parts contiguous at the start of a user tool-result turn
• append images extracted from tool_result content after the full functionResponse sequence
• add a regression test for parallel tool results that include images

Fixes

Fixes #3094

Tests

• cargo test -p antigravity_tools test_parallel_tool_results_with_images_stay_contiguous -- --nocapture
• cargo test -p antigravity_tools test_complex_tool_result -- --nocapture
• cargo test -p antigravity_tools test_thinking_mode_auto_disable_on_tool_use_history -- --nocapture
• git diff --check -- src-tauri/src/proxy/mappers/claude/request.rs