feat(openai-compat): honor tool_choice on /v1 passthrough

[Hrithik-Gavankar]

Summary

• Map OpenAI tool_choice (auto / none / required / specific function) to AI SDK toolChoice when openai_compat tools mode is passthrough (DR-032 / DR-046)
• Reject invalid tool_choice, required/specific-function without usable tools, and unknown function names with 400 invalid_request_error
• Leave tool_choice ignored when tools mode is off (same secure default as tools)
• Document behavior in CONFIGURATION.md and record DR-046 (renumbered: main already used DR-045 for VS Code webview UX)

Closes #77

Rebase note

Rebased onto main to resolve docs/decisions.md conflict (DR number collision). Tool_choice decision is now DR-046.

Test plan

• Unit: mapOpenAIToolChoice / validateToolChoiceAgainstTools cover all shapes + invalid inputs
• Integration: passthrough forwards none / required / specific function; off mode ignores tool_choice
• Integration: invalid tool_choice, required without tools, and unknown function → 400
• AI SDK wiring: streamText receives toolChoice when tools are present
• Manual (optional): with openai_compat.tools: passthrough, call /v1/chat/completions from Open WebUI / curl with tool_choice: "none" and "required" and confirm model behavior

[cidrblock]

Rebased onto main to clear the merge conflict.

Conflict was a DR number collision in docs/decisions.md: main already shipped DR-045 (VS Code webview UX). This PR’s tool_choice decision is now DR-046 (code/docs comments updated accordingly).

Force-pushed: 4fc1006 → 4c35824. Local openai-compat tests: 79 passed.