test: cross-version compatibility check against the last published extension (#2914)

# Why
- Runtime negotiation is only unit-tested against a faked
`__stagehand_runtime` marker. Nothing checks that the current SDK
actually negotiates with the extension we last published, or that an
incompatible runtime fails fast instead of hanging until the 60 s init
timeout.

# What changed
- New
`packages/sdk-ts/tests/browser-runtime/crossVersionCompat.test.ts`:
- `npm pack`s the last published `@browserbasehq/stagehand` (override
with `STAGEHAND_COMPAT_BASELINE_VERSION`), extracts its bundled
extension, and caches it under `node_modules/.cache`.
- Launches local Chrome with that extension through the real
`localBrowser.launch` path and asserts the outcome
`checkProtocolCompatibility` predicts, so the expectation moves when the
protocol version bumps.
- Forks the extension with its marker rewritten to the next protocol
major and asserts `StagehandRuntimeIncompatibleError`
(`protocol-major-mismatch`) within 5 s.
- No CI changes: the existing TypeScript browser job already runs this
directory.

# Test plan
- `pnpm --filter ./packages/sdk-ts test:browser` locally: both cases
pass (~5 s cold, includes `npm pack`).
- Negative control: with the pre-#2912 polling behaviour the synthetic
case fails at the 60 s init timeout, so the assertion catches the
regression.