`session.create` fails with "fetch failed" in SDK headless mode behind corporate proxy (v1.0.36)

### Describe the bug

When using `@github/copilot-sdk` v0.3.0 (CLI v1.0.36) in headless mode behind a corporate HTTP proxy, `session.create` fails with "fetch failed". The CLI subprocess receives all proxy env vars correctly, and standalone `undici` 7.22 can reach `api.github.com` through the same proxy from the same container — but the CLI's internal fetch ignores the proxy.

This is a regression from CLI v0.0.394 where the same setup worked.

### Key evidence

1. **Standalone undici works** — from inside the Docker container:
```
undici version: 7.22.0
Status: 200
Body: {"login":"...", "copilot_plan":"copilot_for_business"...}
```

2. **curl works** — through the same proxy:
```
curl -x http://proxy:443 https://api.github.com/copilot_internal/user → 200
```

3. **CLI subprocess gets proxy env** — confirmed by intercepting `child_process.spawn`:
```
SPAWN command: node /app/node_modules/@github/copilot/index.js --headless --no-auto-update
SPAWN HTTPS_PROXY: http://proxy:443
SPAWN HTTP_PROXY: http://proxy:443
SPAWN NODE_TLS_REJECT_UNAUTHORIZED: 0
SPAWN env: COPILOT_SDK_AUTH_TOKEN = gho_xxx...
```

4. **CLI fails anyway:**
```
FAILED: Request session.create failed with message: fetch failed
```

5. **No TLS MITM** — the proxy does transparent CONNECT (real `*.github.com` cert from Sectigo, not a proxy-injected cert):
```
subject=CN = *.github.com
issuer=C = GB, O = Sectigo Limited, CN = Sectigo Public Server Authentication CA DV E36
```

6. **NODE_EXTRA_CA_CERTS doesn't help** — tested with the full cert chain, same result

### Version comparison

| Component | Working | Broken |
|-----------|---------|--------|
| `@github/copilot-sdk` | 0.1.19 | 0.3.0 |
| `@github/copilot` CLI | 0.0.394 | 1.0.36 |
| Node.js | v22.22.0 | v22.22.0 |
| Result | Sessions work | "fetch failed" |

### Affected version

GitHub Copilot CLI 1.0.36

### Steps to reproduce the behavior

1. Run in a Docker container with `HTTPS_PROXY` set to a corporate HTTP CONNECT proxy
2. Install `@github/copilot-sdk@0.3.0` + `@github/copilot@1.0.36`
3. Create a CopilotClient with `gitHubToken` and start it:
```javascript
const { CopilotClient } = require("@github/copilot-sdk");
const client = new CopilotClient({ gitHubToken: token, useLoggedInUser: false });
await client.start();
```
4. Call `createSession` with `gitHubToken`:
```javascript
const session = await client.createSession({
model: "gpt-4o",
gitHubToken: token,
streaming: false,
systemMessage: { content: "Be brief." },
onPermissionRequest: () => ({ result: "allow" }),
});
```
5. Observe: `Request session.create failed with message: fetch failed`

### Expected behavior

`session.create` should respect `HTTPS_PROXY` / `HTTP_PROXY` environment variables for its internal auth fetch to `api.github.com`, the same way standalone undici 7.22 does from the same container.

### Additional context

- **Environment:** Docker (`node:22-alpine`), Linux x86_64
- **Proxy:** Corporate HTTP CONNECT proxy on port 443, no TLS interception
- **Use case:** Server-side SDK integration for automated coding agent (not interactive CLI)
- **The specific failing path in CLI:** `resolveSessionAuth()` → internal token validation fetch to `api.github.com/copilot_internal/user` — this fetch does not go through the proxy
- **Startup auth also fails silently:** `client.start()` → `trySdkTokenLogin()` → same fetch failure, but caught silently. CLI boots with no auth. The session-level auth then fails loudly.
- **BYOK provider works:** Using `provider: { type: "openai", baseUrl: "https://api.business.githubcopilot.com", bearerToken: token }` in `createSession` bypasses the failing fetch and works — but only for GPT models (Claude/Gemini not available via this endpoint)
- **Related issues:** #333, #2395, #869
- **Workaround:** Downgrade to `@github/copilot-sdk@0.1.19` / `@github/copilot@0.0.394`

Start at the CLI paths named in the report: resolveSessionAuth() and trySdkTokenLogin(), focusing on their internal fetch to api.github.com/copilot_internal/user. Reproduce the failure with the listed Docker, proxy, and version setup, then compare it with standalone undici behavior and the working CLI version. Done means session.create succeeds through HTTP_PROXY or HTTPS_PROXY without requiring the downgrade workaround.

索引モデルが issue の本文から書いたものです。