Fix enableCodeAnalyzersOnTestApps not disabling customCodeCops in workspace compilation (#2323)

## Why

Under workspace compilation (`workspaceCompilation.enabled: true`),
setting `enableCodeAnalyzersOnTestApps: false` was supposed to stop all
analyzers from running against test and BCPT test apps. It correctly
disabled the built-in cops (CodeCop, AppSourceCop, PTECop, UICop) but
left custom analyzers (`customCodeCops`) running, so custom analyzer
DLLs kept producing diagnostics against test apps. This contradicted the
documented behavior and the classic `Run-AlPipeline` container flow.

## What changed

In `Actions/CompileApps/Compile.ps1`, the `CustomAnalyzers` build
parameter is now cleared alongside `Analyzers` in both the test-folder
and BCPT-test-folder branches when `enableCodeAnalyzersOnTestApps` is
false. Previously only `Analyzers` was reset; `CustomAnalyzers` was
computed once before the compile steps and never cleared.

`$buildParams` is a shared mutable hashtable and the regular app-folder
branch compiles first, so clearing the analyzers in the test branches
does not affect normal app compilation.

## Tests

Added two regression tests to the existing `CompileAppsInWorkspace
argument construction` suite in `Tests/CompileFromWorkspace.Test.ps1`,
verifying the module emits `--customanalyzers` when custom analyzers are
supplied and omits it when the list is empty. The full
argument-construction suite (8 tests) passes locally.

No settings schema or `Scenarios/settings.md` changes were needed since
this aligns the behavior with the already-documented meaning of the
setting.

Fixes: #2319

Co-authored-by: aholstrup1 <aholstrup1@users.noreply.github.com>