…n step

The azure-pipelines-PR.yml 'Detect CI VS Roslyn version' step and the
FSHARP_APEX_ROSLYN_VERSION override in eng/Versions.props were committed, but the
script itself was left untracked, so on CI 'powershell: eng\SetApexRoslynVersion.ps1'
could not find the file and failed with 'The module ''eng'' could not be loaded'.

Co-authored-by: Copilot <[email protected]>
Copilot-Session: fe2b3239-4176-4f13-b6f6-486c8799765a

main (via darc) bumped the dotnet/runtime System.* packages to 10.0.8 (assembly
10.0.0.8). The F# VSIX then references System.Composition.AttributedModel 10.0.0.8,
which the CI scout VS (18.7, ships 8.0.0.0) cannot bind -- its binding-redirect
ceiling is below 10.0.0.8 -- so every FSharp.Editor MEF part fails to load,
IFSharpWorkspaceService is null, and all Apex tests fail with an NRE in
LanguageService.fs. This is the same forward-pinned-dependency vs older-CI-VS
problem already handled for Roslyn.

Extend the FSHARP_APEX_ROSLYN_VERSION-gated override in eng/Versions.props to also
roll System.Composition and System.Collections.Immutable back to 10.0.2 for the
Apex leg only. Verified: with a simulated 10.0.8 base the override forces 10.0.2,
and FSharp.Editor compiles and references System.Composition.AttributedModel
10.0.0.2. The committed pin and product/main builds are unaffected.

Co-authored-by: Copilot <[email protected]>
Copilot-Session: fe2b3239-4176-4f13-b6f6-486c8799765a

… leg"

This reverts commit c0c3dd7.

Scaffolds an internal DevDiv DartLab pipeline (modeled on dotnet/roslyn's
azure-pipelines-integration-dartlab.yml) that installs a matching Visual Studio via
the VS bootstrapper on a VS-Platform test machine, deploys the F# VSIX, and runs the
existing Apex tests. This is the robust fix for the version-skew that blocks running
the VSIX on the older pre-baked public scout VS (the VS-SDK floor, e.g. RpcContracts
requiring System.Composition >= 10.0.8, prevents downgrading to match it).

Files (all trigger:none / inert until registered):
- azure-pipelines-integration-dartlab.yml: entry, extends the VS real-sign template.
- eng/pipelines/apex-integration/{stage,integration-job}.yml: provision + install VS +
  deploy VSIX + run Apex via the existing -testApex path.
- eng/pipelines/apex-integration/README.md: external prerequisites + a concrete request
  to the DartLab/VS team.
- eng/setup-pr-validation.ps1: PR-branch setup on the test machine (adapted from Roslyn).

Every access-dependent value is marked '# TODO(P0):'. Requires DevDiv/DartLab access,
an internal dotnet-fsharp mirror, and a VS-Platform pool before it can be registered
and run; those are outside this repo.

Co-authored-by: Copilot <[email protected]>
Copilot-Session: fe2b3239-4176-4f13-b6f6-486c8799765a