CI: pin the .NET 10 SDK to 10.0.402 while aka.ms links resolve to a host that answers 409 #461

Problem

Since about 19:00 UTC on 2026-10-25, every CI job that sets up .NET 10 fails at "Setup .NET" before any of our code runs. That is every job on PRs into release/10.x (#454, #458, #459, #460) and the Build (10.x) docs job on PRs into main. It blocks the 10.3.0 merges, and the v10.3.0 tag's release.yml would fail the same way.

release/10.x's global.json asks for 10.0.100 with rollForward: latestFeature, so actions/setup-dotnet resolves "latest 10.0" through an aka.ms link. That link now redirects to dotnetorigin.blob.core.windows.net, which answers HTTP 409 for every version, including 10.0.402:

dotnet-install: Attempting to download using aka.ms link https://dotnetorigin.blob.core.windows.net/dotnet/Sdk/10.0.403/dotnet-sdk-10.0.403-linux-x64.tar.gz
dotnet-install: Download attempt #1 has failed: 409 Unable to download …
##[error]Failed to install dotnet, exit code: 1. dotnet_install: Error: Could not find `.NET Core SDK` with version = 10.0.403

Given an exact version, the script uses its primary link on builds.dotnet.microsoft.com instead, which answers 200. That is why main's exact 11.0.100-rc.2.26504.105 keeps installing. The last green release/10.x run (18:07 UTC) installed through builds.dotnet.microsoft.com. Microsoft's release metadata still lists 10.0.402 as the latest SDK, so this looks like a half-published 10.0.403.

Change

Pin the .NET 10 SDK in CI to dotnet-version: 10.0.402, the SDK of the last green run, instead of resolving it from global.json:

• release/10.x: the four Setup .NET steps in dotnet.yml and the two in release.yml.
• main: the Setup .NET (release/10.x) step in docs.yml.

global.json stays as it is, so local builds are unaffected. Remove the pin once aka.ms resolves to a working host again.

Test

Each PR's own CI is the check: its "Setup .NET" steps install 10.0.402 and the jobs run.