internal/output/ had zero unit tests; the envelope shape it produces
is consumed by every agent integration on the other side of stdin, so
locking it down with a small test file is high-leverage.
Six tests in errors_test.go:
- TestAsError_Nil, TestAsError_PreservesKnownCode,
TestAsError_UnwrapsThroughFmtErrorf,
TestAsError_PlainErrorBecomesUnknown — pin AsError's contract
against nil / direct / wrapped / plain inputs.
- TestEmitError_EnvelopeShape — round-trips through bytes.Buffer and
asserts exactly two keys ("code", "error"), correct values, and a
trailing newline.
- TestWrap_PreservesCauseForErrorsIs — confirms errors.Is and
errors.As walk through Wrap so future sentinel introductions work.
A comment block documents why EmitError's encode-failure fallback
isn't exercised — json.Marshal of {Code, Message} cannot fail, so the
branch is unreachable from outside the package.
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