* feat(sse): selective upstream 4xx error passthrough util
* feat(sse): relay upstream 4xx error bodies verbatim on the Anthropic request path
createErrorResult() gains an opt-in 7th param opts.passthrough; when set and
the upstream body is eligible (per shouldPassthroughUpstreamError), the
returned response body is the upstream 4xx body verbatim instead of the
sanitized wrapper. Internal classification fields (error/rawMessage/
errorType/errorCode) are never affected.
Wired into chatCore.ts's 7 upstream-error createErrorResult call sites
(model_unavailable / context_overflow / generic upstream-error branches),
gated on sourceFormat === FORMATS.CLAUDE so only /v1/messages requests get
the verbatim body — OpenAI-format paths are unchanged.
* test(quality): register upstream-error-passthrough in stryker tap.testFiles
`tests/unit/upstream-error-passthrough.test.ts` covers `open-sse/utils/error.ts`,
an instrumented module, so `check:mutation-test-coverage --strict` requires it in
`tap.testFiles` — the gate flagged the drift on this PR.
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