[Repo Assist] fix: auto-detect DOTNET_ROOT from symlink-resolved dotnet binary path

[github-actions]

🤖 This PR was created by Repo Assist, an automated AI assistant.

Closes #2122

Problem

On Ubuntu (and other distributions where dotnet is installed via system packages), the dotnet binary is accessed through a symlink:

/usr/bin/dotnet  →  /usr/lib/dotnet/dotnet

When FSharp.dotnetRoot is not configured and DOTNET_ROOT is not set in the environment, FSAC falls back to its hard-coded default of /usr/share/dotnet (the Microsoft install location). This path does not exist on Ubuntu, so VSTestWrapper.tryFindVsTestFromDotnetRoot throws a Win32Exception and test discovery fails entirely — even though dotnet is perfectly available on PATH.

The workaround (manually setting FSharp.dotnetRoot to /usr/lib/dotnet) works but should not be necessary.

Fix

After resolving the dotnet binary path that was already detected via which dotnet, we call Node's fs.realpathSync to follow any symlinks. If the resolved path differs from the original (i.e. dotnet was a symlink), we set DOTNET_ROOT to the parent directory of the real binary before launching FSAC.

Priority order for DOTNET_ROOT propagation:

1. Already in env → leave untouched (no change to existing behaviour)
2. FSharp.dotnetRoot configured → propagate that value unchanged
3. Dotnet binary is a symlink → DOTNET_ROOT = dirname(realpathSync(dotnet))
4. Dotnet binary is not a symlink → do not override (FSAC can handle it)

The symlink-only condition (point 3) avoids incorrectly setting DOTNET_ROOT to a generic bin directory (e.g. /usr/bin) on systems where dotnet is not installed via a symlink.

Trade-offs

• The realpathSync call is synchronous and fast (single syscall), adding negligible overhead during FSAC startup.
• The try/with guard ensures that if realpathSync throws for any reason, we fall back gracefully.
• This supersedes PR [Repo Assist] Fix: propagate FSharp.dotnetRoot as DOTNET_ROOT environment variable to FSAC #2136 by also handling the case where FSharp.dotnetRoot is not explicitly configured. If [Repo Assist] Fix: propagate FSharp.dotnetRoot as DOTNET_ROOT environment variable to FSAC #2136 is merged first, this PR will conflict on the adjacent lines and will need a minor rebase.

Test Status

Full compilation requires the full Fable + webpack toolchain (network not available in this environment). The change uses only existing Node.js APIs (fs.realpathSync, path.dirname) and the emitJsExpr Fable utility already used elsewhere in the codebase. Fantomas formatting check: ✅ passed.

Platform coverage: The fix is conditional on resolvedDotnet <> dotnet so it is a no-op on macOS and Windows where dotnet is not accessed via symlinks, and only activates on Linux distributions where it is needed.

Generated by Repo Assist · ◷

To install this agentic workflow, run

gh aw add githubnext/agentics/workflows/repo-assist.md@4957663821dbb3260348084fa2f1659701950fef