Defense-in-depth: cap readline() so a malformed or maliciously-large single-line file cannot blow up memory before the value is sanitized.
Adds tests-unit/deploy_environment_test.py covering: missing file fallback, basic read, whitespace strip, multi-line (only first line used), empty + whitespace-only files, control-char stripping (header-injection protection), non-ASCII stripping, 128-byte read cap, cache stickiness, and OSError fallback.
Amp-Thread-ID: https://ampcode.com/threads/T-019df26e-96f4-7518-94da-0e4263680e3c
Co-authored-by: Amp <amp@ampcode.com>
