Filter 16: translate tool_choice for Chat Completions

[leseb]

Parent

Part of #35 (responses_to_chat_completions)

Context

The Responses API supports tool_choice variants that don't exist in Chat Completions:

Responses API	Chat Completions equivalent
"auto"	"auto"
"none"	"none"
"required"	"required"
{"type":"function","name":"X"}	{"type":"function","function":{"name":"X"}}
{"type":"allowed_tools","mode":"..."}	No equivalent
{"type":"mcp","server_label":"..."}	No equivalent
{"type":"<hosted_tool>"}	No equivalent

Scope

• Map translatable variants directly
• Define fallback behavior for Responses-only variants (allowed_tools, mcp, hosted tool types)
• Options: map to "auto", strip, or reject
• Consume tool_parse metadata (tool_choice, tool_choice_type) for the decision

Reference

• tool_parse (tool_parse: parse and register function/built-in tools #42) promotes tool_choice and tool_choice_type to metadata and filter results
• translation/chat_completions.rs already handles basic tool_choice mapping