[wasm] Pin linux/win V8 to 15.1.103 for exnref support

[DrewScoggins]

Problem

WASM perf runs (and other V8 shell-engine legs) abort at startup:

Warning: unknown flag --experimental-wasm-exnref.
MONO_WASM: Assert failed: This browser/engine doesn't support WASM exception handling.
main.mjs: Error: Top-level await promise never resolved

Root cause

• [browser] WASM exception handling to use the standardized exnref  #129851 ([browser] WASM exception handling to use the standardized exnref) switched the runtime to the exnref exception-handling proposal, which requires a V8 that understands --experimental-wasm-exnref (V8 15.1+). That PR set linux/macos/win_V8Version to 15.1.103.
• The later automated chrome-for-testing bump [wasm] Bump chrome for testing - linux: 149.0.7827.200, windows: 149.0.7827.201 #129934 recomputed the V8 version from the pinned Chrome (149 → V8 14.9) and reverted linux_V8Version and win_V8Version back to 14.9.207 (leaving only macos_V8Version at 15.1.103).

14.9.207 predates exnref, so it rejects the flag and the runtime's startup feature check (featureWasmFinalEh) asserts. The wasm perf leg is hit because dotnet/performance run_performance_job.py reads linux_V8Version and installs exactly that build via jsvu.

Fix

Restore linux_V8Version and win_V8Version to 15.1.103 (matching macos_V8Version). The v8-linux64-rel-15.1.103.zip and v8-win32-rel-15.1.103.zip binaries exist on the V8 canary CDN that jsvu / DownloadAndInstallV8 use (verified 200 OK), and 15.1.103 supports exnref behind the --experimental-wasm-exnref flag already emitted by eng/testing/WasmRunnerTemplate.sh and the perf harness (dotnet/performance#5247).

Note on the auto-updater

The chrome-for-testing auto-bump derives the V8 version from the Chrome milestone, so a future run will revert this to 14.9.x again unless the pinned Chrome is advanced to a milestone whose V8 is 15.1+ (Chrome 151) or the updater is taught not to downgrade V8 below the exnref floor. Tracking: #129849.

[Copilot]

Pull request overview

This PR updates the WASM test/perf tooling configuration to pin the Linux and Windows V8 versions back to 15.1.103, aligning them with macOS, so V8-based legs can run with the --experimental-wasm-exnref flag required by the standardized WASM exception-handling (exnref) path.

Changes:

• Update linux_V8Version from 14.9.207 → 15.1.103.
• Update win_V8Version from 14.9.207 → 15.1.103.
• Keep macos_V8Version at 15.1.103 (now all OSes match).

[ilonatommy]

@DrewScoggins we added #129974 to prevent any downgrades of this kind in the future.

[DrewScoggins]

I see that there are three failures. One the job got deadlettered. One has this error:

• rm -rf /var/folders/pj/9qcdcssn0jd88t59py0n9jnc0000gr/T/tmp.3YydGWXjNw
• dotnet dev-certs https
  There was an error saving the HTTPS developer certificate to the current user personal certificate store.
• export _commandExitCode=2
• _commandExitCode=2`

And the last seems to have a bad docker image. Are these known issues for WASM?

[DrewScoggins]

Re-running the failed jobs, to see if they pass. Most of the failures looked unrelated.