copilot_chat: Serialize ToolChoice::Any as "required" instead of "any"

[Vastargazing]

Observed behavior: Inline assistant failed for GPT-4.1,
Gemini, and other non-Anthropic models. Claude worked correctly because
Anthropic's API accepts "any" as a valid value.

Fix

Renamed ToolChoice::Any → ToolChoice::Required in both
copilot_chat.rs and responses.rs, matching the convention used by
other OpenAI-compatible providers (open_ai, lmstudio, open_router).

copilot_chat::ToolChoice is a wire type only for the /chat/completions
path — Anthropic models go through into_anthropic() and never touch it,
so no per-model logic is needed.

Also fixes the same serialization bug in responses::ToolChoice, which
was not covered by the original approach, and adds regression tests for
both.

Affected models

• gpt-4.1 via copilot_chat provider
• gemini-* via copilot_chat provider
• Likely affects all OpenAI-compatible models routed through copilot_chat

Screenshots

Bug (only Claude works, Gemini and GPT-4.1 fail):

Fix:

Result: After the fix, all models work correctly via inline assistant.

Release Notes

• Fix inline assistant 400 errors for GPT-4.1, Gemini, and other
  non-Anthropic models via the copilot_chat provider (tool_choice was
  sending "any" instead of "required")

[zed-industries-bot]

	Warnings
⚠️	This PR is missing release notes. Please add a "Release Notes" section that describes the change: Release Notes: - Added/Fixed/Improved ... If your change is not user-facing, you can use "N/A" for the entry: Release Notes: - N/A

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[benbrandt]

Wouldn't this need to be per model then?

[Vastargazing]

After looking at other OpenAI-compatible providers (open_ai, lmstudio, open_router) — they all use a Required variant instead of Any. copilot_chat::ToolChoice is a wire type only for the /chat/completions path; Anthropic models go through into_anthropic() and never touch it, so per-model logic isn't needed.

Updated the PR: renamed Any → Required in both copilot_chat.rs and responses.rs (the latter had the same bug and wasn't covered by the original commit).

Also added regression tests for the serialization output.

[benbrandt]

Right but copilot supports anthropic models as well, correct?

[Vastargazing]

Yes, but Anthropic models are routed through a completely separate path. In stream_completion, the first check is model.supports_messages() — Anthropic models all have the /v1/messages endpoint and go through into_anthropic(), which uses anthropic::ToolChoice. That type serializes Any as "any" (with rename_all = "lowercase", tagged with type), which is exactly what Anthropic's API expects.