fix(core): support Opinion code/msg envelopes

[realfishsam]

Summary

• Accept Opinion REST envelopes using documented code/msg fields while preserving legacy errno/errmsg compatibility.
• Apply the same error-code normalization to Opinion order submission and cancellation responses.
• Add targeted fetcher tests for documented success/error envelopes and legacy compatibility.

Fixes #416

Test Plan

• npm test --workspace=pmxt-core -- --runTestsByPath test/exchanges/opinion-fetcher-envelope.test.ts --runInBand

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Updates Opinion API envelope handling to accept both documented code/msg envelopes and legacy errno/errmsg envelopes.

Blast Radius

Opinion fetcher/index plus targeted envelope Jest tests.

Consumer Verification

Before (base branch):
Base treated errno as required, so a documented {code:0,msg:"success"} success envelope could be misclassified because data.errno !== 0 when errno is undefined.

After (PR branch):
PR computes code ?? errno, uses msg || errmsg, and adds tests for success/error/legacy envelopes. Core build/Jest passed (25 suites, 647 tests); root verification then hit missing pytest.

Test Results

• Build: PASS
• Unit tests: CORE PASS (25 suites, 647 tests; root verification blocked by missing pytest)
• Server starts: PASS during root verification
• E2E smoke: NOT VERIFIED against live Opinion API

Findings

No blocking findings.

PMXT Pipeline Check

• Field propagation (3-layer): N/A
• OpenAPI sync: N/A
• Financial precision: N/A
• Type safety: OK
• Auth safety: OK

Semver Impact

patch -- venue response-envelope bug fix.

Risk

Live Opinion API behavior was not sampled; mocked envelope tests cover the concrete drift.