fix: surface API errors in SDK streams

[ch-z-hc]

Bug

The shared SDK SSE iterator yields every JSON payload as a successful event, including server error frames delivered after HTTP 200. A consumer can finish a failed text or speech generation without an exception, or receive an API error as a transcription delta and later lose its cause to the generic incomplete-transcript error.

The Anthropic streaming contract includes in-stream error events; MiniMax's text endpoint uses this compatible format. The CLI audio decoder already checks both error and nonzero base_resp.status_code, but the SDK shared iterator does not.

Fix

Check both error shapes before yielding and reuse mapApiError, including the response URL for endpoint-specific hints. Keep JSON parsing in its own catch so quota/content-filter errors are not relabeled as parse failures. Preserve previous valid chunks, successful status code 0, empty events and [DONE] handling. No new dependency or public API.

This is separate from #279 (response-body ownership/cleanup). It does not duplicate that cleanup change or claim to solve it.

Validation

• Five new regressions through the public text, speech synthesis and transcription SDK methods, using loopback HTTP/SSE fixtures. Unchanged product code with the same tests: 43 passed / 5 failed. Patched SDK, HTTP, SSE parser and audio decoder tests: 96 passed / 0 failed.
• Production SDK bundles against the same local fixture: original bundle silently completes all three error scenarios; patched bundle rejects all three with quota exit code 4, preserving only the earlier valid event. All three successful controls still complete. No live model requests.
• Two independent automated blind reviews: no actionable findings. Additional reviewer probes cover malformed JSON, empty/DONE events, successful status code 0 and business-error mappings. These are automated reviews, not human approval.
• Windows and Linux typecheck, lint and CLI/SDK/declaration builds pass. Lint retains the existing explicit-any warning in test/sdk/speech.test.ts.
• Full Linux suite, same added tests on both versions: unchanged main 596 passed / 8 failed; patched 601 passed / 3 failed. The five new failures are fixed. The remaining three fail on both versions: two unreachable-region timeout cases and the FileSDK upload base-URL assertion. Those failures were not diagnosed or changed; the full suite is not green.

Full-suite validation uses temporary configuration, a process-local no-op URL opener and a validation-only fetch preload rejecting external hosts by default (test-owned mocks can override it). Windows OAuth/full-suite tests were not run because of the existing browser-opening mock issue. No existing assertions, timeouts or skip conditions were relaxed.

Implemented and tested with Codex assistance.

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