[0.42-r2 M7] parity contract test matrix is thin — extend to error/JSON/405 shapes

[SisyphusZheng]

Severity: medium · Category: test gap

Evidence. request-time-parity.test.ts asserts 6 groups. Round-2 review asks for the error/negotiation family: fetch-header 404 JSON, action-throw 500 (dev detail vs prod scrub divergence is EXPECTED — assert the documented divergence honestly), Response-contract-violation 500, 405, and the 303→GET redirect chain.

Fix. Extend the parity test with these groups, marking intentional dev/prod divergences explicitly.

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Filed from the 0.42 round-2 review (docs/audit/2026-07-28-alpha5-round2-review.md). Target line: 0.42.0-alpha.6.

[SisyphusZheng]

Fixed and shipped in v0.42.0-alpha.6 (npm dist-tags latest/alpha at 0.42.0-alpha.6, tag ccc9407). The fix and its regression test landed via PR #594 (fixture suite 49 tests × 3 engines = 147 green, including the new round-2 specs; unit + parity suites green).