gateway: 400 when a required tool choice leaves nothing callable - #169

Checked through the gateway: Responses allowed_tools required with an unknown/empty list and Gemini ANY with an unknown allowedFunctionNames are refused with a 400 in the client's own shape before any upstream call; auto, an existing named tool, passthrough, remaining web search, AUTO with a list and ANY without one are all untouched. Nice.

Two things before merging:

1. Scope is wider than the PR says. The new check after parse in translate applies to every translated request, not just allowlist filtering. These now 400 where main forwarded without tools and got a text reply:

   • Anthropic tool_choice:{type:"any"} with only non-search server tools (e.g. web_fetch_20250910)
   • Chat tool_choice:"required" with only custom tools
   • Responses required with only custom/mcp tools (Codex's apply_patch is custom)

   Please either limit the check to the allowlist cases (Responses allowed_tools, Gemini ANY + allowedFunctionNames), or keep it broad but say so and add tests for those cases. I'd lean towards limiting it — refusing a Codex request that forces a custom tool is riskier than the silent drop.

2. Routes that don't go through translate — Kiro (kiro.go), Cursor (cursor.go), Devin (devin.go) and the Claude subscription route (claude_subscription.go) parse the request themselves, so Gemini ANY with an unknown name still goes out as required-with-no-tools there. Putting the check where all of them see it (or in the shared parse helper) would cover them.

Thanks! Re-reviewed the new head line by line and ran it end to end through the gateway against scripted upstreams — all the cases from my previous review now pass, and the new tests fail without the change. Merged; ships in v0.1.350. (#168 had been closed at the same moment as your update comment, which looked accidental, so I reopened it to merge.)