fix: preserve tool calls when thinking models return no text content (#11866)

* fix: preserve tool calls when Qwen3-Coder returns thinking content without text

Two bugs caused Qwen3-Coder (and similar thinking models) to silently
drop tool calls:

1. Ollama: When the model produces thinking content and tool calls but
   no text content, the early return in convertChatMessage only yielded
   the thinking message, discarding tool calls entirely.

2. OpenAI/vLLM: fromChatCompletionChunk used an if/else chain where
   content was checked before tool_calls, so any chunk with both fields
   would lose its tool calls.

Fixes #8744

* revert: remove speculative openaiTypeConverters change

The vLLM "list index out of range" error is server-side (Python), not
fixable by reordering client-side chunk parsing. Keep only the Ollama
fix which addresses the confirmed bug.