diagnostics_channel: return original thenable

[Qard]

This makes tracePromise return the original thenable to allow custom thenable types to retain their methods rather than producing the chained result type.

This branches the behaviour between native promises and thenables such that native promises retain the correct unhandledRejection behaviour while thenables produce the correct original return value and so retain methods present on that type.

I think that because native promises are consistently shaped it doesn't actually matter that it remains chained rather than branched.

cc @nodejs/diagnostics

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.77%. Comparing base (efa05f2) to head (3522e89).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #62407   +/-   ##
=======================================
  Coverage   89.77%   89.77%           
=======================================
  Files         697      697           
  Lines      215773   215782    +9     
  Branches    41304    41300    -4     
=======================================
+ Hits       193704   193713    +9     
+ Misses      14162    14158    -4     
- Partials     7907     7911    +4     

Files with missing lines	Coverage Δ
lib/diagnostics_channel.js	98.16% <100.00%> (+0.02%)	⬆️

... and 32 files with indirect coverage changes

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• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
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[Renegade334]

Does this change have a specific case in mind? The A+ standard mandates .then() to return another "promise" instance (either the same object or a new object at the implementation's discretion), so any custom features of a compliant thenable should also be present on the object returned by .then()?

[Qard]

No, that is not the case. A thenable can return any variety of promise, not necessarily another of itself. It's most common that a thenable returns native promises from its then/catch, so you lose access to whatever methods it had when following the chain.

[github-actions]

Failed to start CI

- Validating Jenkins credentials
✔  Jenkins credentials valid
- Querying data for job/node-test-pull-request/72280/
[SyntaxError: Unexpected token '<', ..."    
  https://github.com/nodejs/node/actions/runs/27762715319

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74236/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74268/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74276/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74284/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74305/

[Qard]

Jenkins is passing. Github Actions seems to have an unrelated failure though.

[Renegade334]

Rebasing on a40bb49 would get GHA green, but rather than re-running everything, I suggest we just manually land this instead.

[Qard]

Does commit-queue actually consider GHA failures? I thought it only cared about Jenkins. My thinking was to just use that with that assumption.

[Renegade334]

Does commit-queue actually consider GHA failures?

It would complain, aye.

   ℹ  This PR was created on Mon, 23 Mar 2026 14:06:11 GMT
   ✔  Approvals: 3
   ✘  1 GitHub CI job(s) failed:
   ✘    - aarch64-linux: with shared boringssl-0.20260526.0: FAILURE (https://github.com/nodejs/node/actions/runs/27838891443/job/82414639619)
   ℹ  Last Full PR CI on 2026-06-20T12:05:56Z: https://ci.nodejs.org/job/node-test-pull-request/74305/
✔  Build data downloaded
   ✔  Last Jenkins CI successful
--------------------------------------------------------------------------------
   ✔  Aborted `git node land` session