language_models: Add SuperGrok OAuth provider (#63248)

# Objective

Grok in the agent is BYOK only today (`x_ai` plus an API key from
console.x.ai). SuperGrok is a separate subscription, so you either pay
twice or you skip Grok in Zed. This makes SuperGrok a first-class LLM
provider, same idea as ChatGPT Subscription or GitHub Copilot Chat.

Discussion:
https://github.com/zed-industries/zed/discussions/63114#discussioncomment-18158663

## Solution

Adds a SuperGrok provider next to the API-key one. Sign in / Sign out is
in Settings → AI → LLM Providers, same shape as ChatGPT Subscription.
Tokens go in the OS keychain, not settings.json.

Shipped models: Grok 4.6 (default), Grok 4.5, Grok Build 0.1. Reasoning
effort on 4.5/4.6 matches the BYOK xAI provider (no `none`, default
high, 4.6 has xhigh). Some Grok plans still get HTTP 403 after a
successful login. The error says the account cannot use this API and
points at the API-key provider.

## Testing

I have been running this locally for a few days on macOS 27 (public
beta) with a SuperGrok Heavy subscription. No issues so far.

`cargo test -p x_ai_subscribed --lib` and `cargo test -p language_models
--lib provider::x_ai_subscribed` pass.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase

SuperGrok LLM provider settings:

<img width="956" height="816" alt="Screenshot 2026-08-26 at 13 41 17"
src="https://github.com/user-attachments/assets/ef2d464a-66ad-416a-a924-725a18d41164"
/>

---

Release Notes:

- Added SuperGrok sign-in so a SuperGrok subscription can use Grok
models in the agent

Cargo.toml

@@ -237,6 +237,7 @@ members = [
     "crates/worktree",
     "crates/worktree_benchmarks",
     "crates/x_ai",
+    "crates/x_ai_subscribed",
     "crates/zed",
     "crates/zed_actions",
     "crates/zed_credentials_provider",
@@ -501,6 +502,7 @@ which_key = { path = "crates/which_key" }
 workspace = { path = "crates/workspace" }
 worktree = { path = "crates/worktree" }
 x_ai = { path = "crates/x_ai" }
+x_ai_subscribed = { path = "crates/x_ai_subscribed" }
 zed_actions = { path = "crates/zed_actions" }
 zed_credentials_provider = { path = "crates/zed_credentials_provider" }
 zed_env_vars = { path = "crates/zed_env_vars" }