Drop the deprecated io/ioutil and the stdlib errors alias — the internal
errors package already re-exports Is, As, Unwrap, and Join, so callers
can keep a single import. Add a utils_test.go covering every exported
helper (API response/error/CORS, ParseBody, BuildStreamURL, peer/closed
network classifiers, URL→stream conversion, RTC/SRT packet sniffers,
ICE/SRT parsers, and ParseListenEndpoint) for 96.3% statement coverage.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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