fix(server): preserve tool result adjacency in compat converters

[marksverdhei]

What changed

• emit Anthropic tool-result messages before mixed user text
• apply the same ordering fix to Gemini function responses
• add model-free conversion regression tests for both APIs

Why

OpenAI-compatible message ordering requires a tool result to remain adjacent to the preceding assistant tool call. Both converters collected tool results and appended them after ordinary text from the same source turn, producing assistant -> user -> tool instead of assistant -> tool -> user.

The separate Gemini duplicate-ID behavior for parallel same-name calls is intentionally unchanged because it requires cross-turn response pairing.

Validation

• cmake --build build --target test-chat -j 8
• ./build/bin/test-chat
• git diff --check

Fixes #151