test(grok-local): isolate billing env in usage cost test (#11285)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work.
> - Local adapters report run output, token use, and cost data.
> - The Grok local adapter now reports real token use and cost data.
> - Its new billing test must prove the no-key path and the API-key
path.
> - The no-key assertion used the caller environment without isolation.
> - This made the test fail when `XAI_API_KEY` was already set.
> - This pull request isolates that environment state in the test.
> - The benefit is stable coverage for the cost gate from #10433.

## Linked Issues or Issue Description

Refs #10433

**What happened?**

The Grok local usage and cost test asserted subscription billing while
it still used the ambient process environment. If `XAI_API_KEY` was set
before the test ran, the adapter selected API billing instead. The
subscription assertion could then fail on a developer machine or a CI
runner with provider credentials.

**Expected behavior**

The test should prove the subscription path with no `XAI_API_KEY`. It
should also prove the API billing path with a test key.

**Steps to reproduce**

1. Start from `master` after #10433.
2. Set `XAI_API_KEY` in the shell environment.
3. Run `vitest` for
`packages/adapters/grok-local/src/server/execute.test.ts`.
4. Observe that the subscription half can take the API billing branch
without test isolation.

**Paperclip version or commit**

`master` after #10433.

**Deployment mode**

Built from source.

## What Changed

- Isolated `XAI_API_KEY` with save, delete, set, and restore logic
around both billing assertions.
- Gave the subscription and API billing checks separate run ids and temp
roots.

## Verification

- `XAI_API_KEY=ambient-test-key corepack pnpm exec vitest run
packages/adapters/grok-local/src/server/execute.test.ts`
- `corepack pnpm --filter @paperclipai/adapter-grok-local typecheck`

## Risks

Low risk. This changes test setup only. It does not change Grok local
adapter runtime behavior.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

OpenAI GPT-5 Codex local coding agent. The agent used shell tools,
GitHub CLI, and local test execution. The context window size was not
exposed in this run.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [x] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] All Paperclip CI gates are green
- [x] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge

Co-authored-by: Claude <noreply@paperclip.ing>

packages/adapters/grok-local/src/server/execute.test.ts

@@ -139,8 +139,6 @@ describe("grok_local execute", () => {
   });
 
   it("reports real per-run token usage, marks it as per_run, and only surfaces cost for API billing", async () => {
-    const root = await makeTempRoot();
-
     runProcessMock.mockImplementation(async () => ({
       exitCode: 0,
       signal: null,
@@ -159,8 +157,8 @@ describe("grok_local execute", () => {
       stderr: "",
     }));
 
-    const baseCtx: AdapterExecutionContext = {
-      runId: "run-1",
+    const makeCtx = async (runId: string): Promise<AdapterExecutionContext> => ({
+      runId,
       agent: {
         id: "agent-1",
         companyId: "company-1",
@@ -169,27 +167,30 @@ describe("grok_local execute", () => {
         adapterConfig: {},
       },
       runtime: { sessionId: null, sessionParams: null, sessionDisplayId: null, taskKey: null },
-      config: { cwd: root },
+      config: { cwd: await makeTempRoot() },
       context: {},
       authToken: "run-token",
       onLog: async () => {},
-    };
-
-    // Subscription billing (no XAI_API_KEY): token usage is populated, but
-    // there is no marginal dollar cost so costUsd stays null.
-    const subscriptionResult = await execute(baseCtx);
-    expect(subscriptionResult).toMatchObject({
-      usage: { inputTokens: 2384, outputTokens: 261, cachedInputTokens: 23040 },
-      usageBasis: "per_run",
-      billingType: "subscription",
-      costUsd: null,
     });
 
-    // API-key billing: same token usage, plus the real dollar cost.
     const previousApiKey = process.env.XAI_API_KEY;
-    process.env.XAI_API_KEY = "test-key";
     try {
-      const apiResult = await execute(baseCtx);
+      // Subscription billing (no XAI_API_KEY): token usage is populated, but
+      // there is no marginal dollar cost so costUsd stays null. Clear the key
+      // explicitly so the ambient environment (dev machine or CI with provider
+      // secrets) cannot flip this branch to API billing.
+      delete process.env.XAI_API_KEY;
+      const subscriptionResult = await execute(await makeCtx("run-subscription"));
+      expect(subscriptionResult).toMatchObject({
+        usage: { inputTokens: 2384, outputTokens: 261, cachedInputTokens: 23040 },
+        usageBasis: "per_run",
+        billingType: "subscription",
+        costUsd: null,
+      });
+
+      // API-key billing: same token usage, plus the real dollar cost.
+      process.env.XAI_API_KEY = "test-key";
+      const apiResult = await execute(await makeCtx("run-api"));
       expect(apiResult).toMatchObject({
         usage: { inputTokens: 2384, outputTokens: 261, cachedInputTokens: 23040 },
         usageBasis: "per_run",