fix(proxy): preserve HTTP statuses for streaming errors

[SsuJojo]

Summary

Preserve the actual HTTP error status when a streaming request fails before its SSE response body starts. Previously, the shared SSE error helper emitted an error event but left Hono's default HTTP 200 status, so clients and reverse proxies could treat failures as successful responses.

Changes

• Set the supplied HTTP status before opening the SSE response in src/routes/shared/stream-error-response.ts; this covers direct upstream transport failures (502), proxy errors, and no-account responses (503).
• Add regression coverage for streaming 502/500/503 responses and update the Responses streaming 429 expectation to verify the upstream status is preserved.

Test Plan

• Windows focused unit/integration/E2E tests — 59 passed across 5 files.
• npx tsc --noEmit — passed on Windows.
• npm test — attempted on Windows: 3210 passed, 25 failed, 25 skipped across 13 failed files. Failures include POSIX shell/path incompatibilities (/bin/sh unavailable and C:\... paths passed to bash), Electron hook timeouts, Windows EBUSY cleanup, and invalid-token errors in npm/release-notes tests. Full CI will run on GitHub.

Notes

The Windows full-suite failures are not asserted to be caused by this patch; the changed streaming status tests pass in the focused run.