fix(tests): isolate Vercel sandbox SDK fixture

[joelbrilliant]

Problem

Current main fails all 16 Vercel sandbox environment tests when CI disables lazy installs.

The test fixture injects a fake vercel SDK into sys.modules, but the production lazy dependency loader still checks for the real installed distribution and raises before the fake SDK can be exercised.

Fix

Stub tools.lazy_deps.ensure inside the fake-SDK fixture. Production dependency handling is unchanged.

Verification

• Reproduced on pristine main: 16 failed
• Patched Vercel sandbox tests: 16 passed
• Vercel sandbox and related lazy dependency suites: 83 passed
• Ruff 0.15.10: clean
• git diff --check: clean

[joelbrilliant]

Superseded by #74576, which landed the same root-cause fix on main as 8eb06e75b, with the stub scoped directly to _ensure_vercel_sdk.

Closing this duplicate. Its full CI run was green, and I am rebasing the affected feature PRs onto the upstream fix now.