fix(templates): guard realpath() in with-cloudflare-d1 isCLI check

[nickhaubner]

What

One-character fix in `templates/with-cloudflare-d1/src/payload.config.ts` — add optional chaining to the `realpath()` return value before calling `.endsWith()`:

```diff
-const isCLI = process.argv.some((value) => realpath(value).endsWith(path.join('payload', 'bin.js')))
+const isCLI = process.argv.some((value) => realpath(value)?.endsWith(path.join('payload', 'bin.js')))
```

When the bug surfaces

When the template is deployed unmodified to a Cloudflare Worker and any request hits the Next.js handler (e.g. `GET /admin`):

```
TypeError: Cannot read properties of undefined (reading 'endsWith')
```

Every request returns HTTP 500 with the default Next.js error page; the error is not surfaced in Workers Observability, only in `wrangler tail`.

Root cause

`realpath` is defined as `(value) => fs.existsSync(value) ? fs.realpathSync(value) : undefined` — so it returns `undefined` for paths that don't exist on the FS. On Workers, `process.argv` entries (e.g. `'node'`) don't resolve to real files, so `realpath(value)` is `undefined` and the subsequent `.endsWith()` throws. Locally (in `pnpm dev`) and during `payload migrate` this works because there is a real Node binary at `process.argv[0]`.

Test

After applying this patch to the unmodified template and deploying to Workers, `/admin` returns 200 and the "Create First User" form renders correctly. Reproduced and verified on a fresh degit of `templates/with-cloudflare-d1` at d6b2d73, Wrangler 4.61.1, Workers compat date 2026-05-16, Node 24.

The patched config has been running in production at https://cms.sutraelements.com/admin (private downstream repo at https://github.com/nickhaubner/sutraelements; the fix in our local copy lives in src/payload.config.ts#L18).

Risk

Negligible — optional chaining only changes behavior when `realpath()` returns undefined, in which case the previous code would have thrown. All existing successful paths are unchanged.