ci: exercise DDTest with action-installed tracer on Express

[anmarchenko]

What

Run the real Express/Mocha project with the action-installed Datadog tracer and the latest published DDTest prerelease. Each run selects the newest published, non-draft prerelease by publication time, downloads its Linux amd64 binary and prints its version.

The DDTest action preload workflow verifies that dd-trace is outside the Express project, runs the full suite as a direct Mocha control, then plans and runs the Express tests through DDTest. The DDTest steps do not set NODE_OPTIONS: DDTest must resolve the tracer from the action's DD_TRACE_PACKAGE and add its preload. The direct Mocha control supplies its own preload.

Why

This PR originally reproduced a failure where DDTest could not resolve an action-installed tracer from the project. The workflow now checks the latest DDTest prerelease against the same real project. It follows published prereleases automatically.

The fixture sets DD_TRACE_ROUTER_ENABLED=false for direct Mocha and DDTest execution. Express's own tests assert that middleware and res.format receive the identical next function; dd-trace 6.16.0 wraps that function, causing failures even without DDTest. Only router instrumentation is disabled; Mocha test tracing and every Express test remain enabled. This run validates test tracing and DDTest execution, not router instrumentation.

E2E testing

Prerequisite: DD_API_KEY is configured as an Actions repository secret in anmarchenko/express.

1. Push an update to this PR, rerun its DDTest action preload workflow, or dispatch the workflow from anmarchenko/ddtest-action-preload.
2. Inspect Install latest DDTest prerelease in the job. Expect a successful download and the version reported by the newest published prerelease.
3. Inspect Check the action preload is outside the project. Expect the action's absolute preload to resolve and project-local dd-trace/ci/init to remain unavailable.
4. Expect the direct Mocha control, Plan Mocha tests with DDTest, and Run planned Mocha tests with DDTest to succeed. Confirm both direct and DDTest run logs contain the full passing Express suite, including res.format tests, with no failed tests.

The job runs on a disposable hosted runner; no fixture cleanup is needed. This PR targets the personal fork, not the upstream Express repository.

[coveralls]

Coverage Report for CI Build 36729207663

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Coverage: 99.612%

Details

• Patch coverage: No coverable lines changed in this PR.

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Coverage Regressions

Requires a base build to compare against. How to fix this →

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Coverage Stats

Relevant Lines:	774
Covered Lines:	771
Line Coverage:	99.61%
Coverage Strength:	2600.83 hits per line

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💛 - Coveralls

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-30T14:28:29.304740Z	3f1b2cc	Draft marked ready

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