The LocalEnvironment payload-mode test passed on base: it exercises
base.py/local.py, which this stack does not change, so it guarded nothing
here. The Daytona/Vercel staged-stdin transport had no coverage at all.
Swap it for one Vercel test on the existing fake SDK. It checks that the
160 KiB + NUL/0xFF payload is uploaded byte-exact with mode 0o600, never
appears in run_command argv, and that the script starts with the
exec-redirect prefix. It fails on base, where the payload is dropped. Also
drop the change-detector _stdin_mode assert from the Modal test; the
assertions after it already prove the behaviour. The stack still adds two
tests (Modal + Vercel).
Co-authored-by: JoaoMarcos44 <joaomarcosdias444@gmail.com>
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