Retain complete PHPUnit failure artifacts

[chubes4]

Summary

Retain PHPUnit JUnit XML as a bounded, redacted artifact before runtime teardown on success and failure. Test-results and the manifest reference the artifact with application/junit+xml and SHA-256, while command failure responses remain bounded.

Clear prior reports before each invocation and validation path so a failed or discovery-only command cannot reuse stale testcase evidence. Capture failures retain structured diagnostics. The capture bound applies to retained output; the Playground text-read API does not itself bound allocation before reading.

Verification

• Final head f09b8b5117990c199aa0fced150e2f2322de8742: all five GitHub checks passed.
• Focused structured-evidence, result-path and failure-diagnostics tests passed.
• Real failed-PHPUnit integration verifies complete failure/error identities beyond the response cap, XML metadata/digest, credential redaction and stale-report cleanup.
• Real MDI/DME parity runs used the capture contract to retain all 1,053 testcase identities. Parent verification read and parsed both native and MySQL JUnit artifacts, including a native run with 47 failed/error identities and a passing MySQL control. These results validate evidence retention, not MDI parity.

Reproduction: normal npm ci (including postinstall), npm run build, then npx tsx tests/phpunit-structured-evidence.test.ts, npx tsx tests/phpunit-result-paths.test.ts, and npx tsx tests/playground-phpunit-readonly-cache.integration.test.ts.

Supports Automattic/markdown-database-integration#393 and #377. No release or deployment.

AI Assistance

GPT-6 Astra (openai/gpt-6-astra) and GPT-5.6 Terra (openai/gpt-5.6-terra) via OpenCode implemented and verified the capture changes. Astra reviewed stale-output handling and bounds, independently checked real native/MySQL artifacts and final CI, and finalized the PR under Chris Huber's direction.