[Repo Assist] test: use TcpListener(0) for reliable free-port selection in HTTP test server

[github-actions]

🤖 This is an automated pull request from Repo Assist, an AI assistant for this repository.

Problem

The startHttpLocalServer() helper in Http.fs selected a free port by:

1. Picking a random number from 10000–65000
2. Checking IPGlobalProperties.GetActiveTcpListeners() in a loop until no conflict

This approach has a TOCTOU race (time-of-check to time-of-use): another process can grab the port between the check and Kestrel's bind. On busy Windows CI agents (many parallel processes, ephemeral ports recycled rapidly) this caused intermittent failures — 1 Failed, 2889 Passed — visible in the Windows CI job for PR #1717.

The testMultipartFormDataBodySize tests already carried a Windows-CI-specific Assert.Ignore workaround for this exact issue.

Fix

Replace the random-port-then-check logic with TcpListener(IPAddress.Loopback, 0):

let freePort =
    let listener = new TcpListener(System.Net.IPAddress.Loopback, 0)
    listener.Start()
    let port = (listener.LocalEndpoint :?> System.Net.IPEndPoint).Port
    listener.Stop()
    port

The OS assigns a free port (via the kernel, atomically). The window between Stop() and Kestrel's Bind() is microseconds — negligible in practice and far better than the previous approach.

Also removes the now-unnecessary Windows-CI skip workaround from testMultipartFormDataBodySize, and fixes Dispose() to properly await StopAsync instead of fire-and-forget.

Test Status

• ✅ dotnet test tests/FSharp.Data.Core.Tests/ --filter Http — 136 passed, 0 failed, 0 skipped (multipart tests now run on all platforms without the skip workaround)
• ✅ dotnet test tests/FSharp.Data.Core.Tests/ — 2896 passed, 0 failed

Generated by Repo Assist

Generated by 🌈 Repo Assist, see workflow run. Learn more.

To install this agentic workflow, run

gh aw add githubnext/agentics/workflows/repo-assist.md@4ea8c81959909f40373e2a5c2b7fdb54ea19e0a5