ci: test against the current Node.js nightly major

[ostapondo]

This relates to...

Fixes #5469.

Rationale

Node 25 nightlies stopped in February when main moved on, and we are on
v27 now, but this workflow still asks for 25. So test-shared-builtin
downloads the same February tarball every night, v25.6.1-nightly20260209,
and runs its frozen test suite against current undici. Six of its
client-proxy tests fail because nodejs/node@0c138da5 updated them in July
for the 8.7.0 proxy behaviour, and the February copy still expects
CONNECT.

The 25-nightly test and autobahn jobs sit on those same old builds. They
stay green, so nothing has been testing current nightlies at all.

Changes

Look up the newest nightly major from the download index and pass it to
test, autobahn and test-shared-builtin, so this does not need bumping
again when main moves to 28.

report-failure now also fires when that lookup fails, instead of skipping
the whole suite quietly.

Features

N/A

Bug Fixes

N/A, CI only.

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested. actionlint and yamllint pass, and I checked the lookup
  against the live index: the old code resolves the February tarball,
  the new code resolves today's v27 nightly. I could not run
  test-shared-builtin end to end, since fork runs skip on the
  repository guard.
• Benchmarked, N/A
• Documented, N/A
• Review ready
• In review
• Merge ready

[mcollina]

lgtm

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.45%. Comparing base (4ae29b2) to head (e05ad92).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5654      +/-   ##
==========================================
+ Coverage   93.42%   93.45%   +0.02%     
==========================================
  Files         110      110              
  Lines       38723    38723              
==========================================
+ Hits        36178    36188      +10     
+ Misses       2545     2535      -10     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[ostapondo]

Both failures are from main, not this branch.

CodeQL: #5634 bumped init to 4.37.3 but left autobuild and analyze at
4.36.2, so they choke on the config init writes. #5655 fixes that.

The node 24 one is http2-request-never-settles.js, which dies with SIGSEGV
and no output. That is the maglev crash from nodejs/node#64841, and it hits
main's own runs too.

Will rebase once #5655 is in.