Download zstd for Windows integration tests directly instead of via choco

What changes

The Windows integration test jobs stop running choco install zstandard. They download the zstd 1.5.7 release zip from GitHub, which is the file the chocolatey package downloads anyway, check its SHA-256, extract zstd.exe with the 7-Zip already on the runner, and put it on the path. The download is tried up to five times. Nothing else in the job changes and no release artifact is affected.

Why

In the last 30 days this step failed 138 times. chocolatey.org timed out, or choco crashed after it had already extracted the binary. Those attempts became 94 final job failures and dropped 26 of the 94 merge train pipelines that failed in that window, before any test had run. The team has already pinned the version, added a checksum and logged the exit code. This removes chocolatey.org and choco from the job.

Impact

About a quarter of the merge train pipelines that fail today fail on this step, so for an MR author it is the single most common reason a car is dropped after a full hour of pipeline. For the merge train it is roughly a 39% to 30% failure rate. Nothing changes for a passing job except that it no longer talks to chocolatey.org.

Prediction: Windows integration job failures caused by choco install go from about 94 per 30 days to zero. The merge train pipeline failure rate drops from 39% to about 30%.

How to verify after merge

Compare the 30 days after the merge with the 30 days before: failed Windows integration jobs whose trace mentions choco install (expected: none), and the share of merge train pipelines that fail. A zstd download failed line in a trace means GitHub was unreachable for all five attempts.

On this MR's own pipeline all eight shards downloaded and used zstd from GitHub.

Prepared by an agent from Igor's CI analysis of this project. The prediction is written down so it can be checked against the following weeks of pipelines.