fix(openai): handle mixed content and tool call stream chunks

[slegarraga]

Summary

• Handle OpenAI chat-completion stream chunks that contain both delta.content and delta.tool_calls.
• Add regression coverage so a mixed content/tool-call chunk still emits the tool input and tool call parts.

Root cause

The chat-completion stream parser used a switch over delta content vs. tool calls. When an OpenAI-compatible provider emitted both fields in the same chunk, the content branch ran and the tool-call delta was skipped.

This matches the downstream failure mode described in charmbracelet/crush#2936, where local OpenAI-compatible providers can return finish_reason: "tool_calls" with populated streamed tool-call deltas that never reach the agent loop.

Validation

• go test ./providers/openai -run 'TestDoStream/should handle content and tool call deltas in the same chunk' -count=1
• go test ./providers/openai -count=1
• go test ./providers/openaicompat -count=1
• go test ./... -count=1