test: add an opt-in live xAI proxy test

[TheGreatAxios]

Closes CL-9411

Summary

• Adds e2e/live.test.ts: one streamed text turn through runInference against xAI's CLI chat proxy (XAI_OAUTH_PROXY_BASE_URL), passing the user id decoded by xaiUserIdFromAccessToken as providerOptions.xaiUserId. A token without a JWT sub claim fails the test instead of silently dropping the header.
• Skipped unless XAI_LIVE_ACCESS_TOKEN is set. XAI_LIVE_MODEL defaults to the first of XAI_DEFAULT_MODELS (grok-4.5).
• Adds test:e2e (bun test e2e), sets test to bun test src, adds e2e to the tsconfig include, and removes bunfig.toml, whose [test] root = "src" kept bun test e2e from finding the suite.

Checks

• bun run check: 2 pass. bun run test:e2e: 1 skip.
• Not run against the real proxy (no token). With a placeholder token the suite reaches the proxy and gets Unauthorized, which confirms the wiring.