ScriptAnalysisService.RunInSandboxAsync compiled and executed arbitrary
user C# in the central host process with no trust-model enforcement — the
forbidden-API set was only a Monaco editor diagnostic. A Design-role user
could run System.IO/Process/Reflection/network code on the central node.
Added a Roslyn semantic gate (EnforceTrustModel) invoked after compilation
and before script.RunAsync, and on nested shared scripts in callSharedFunc;
a script referencing any forbidden API is rejected before it runs.
Reworked FindForbiddenApiUsages: it now resolves every identifier against
the semantic model and checks types and members, so a fully-qualified call
(System.IO.File.WriteAllText) is caught — the pre-fix check only inspected
the leftmost identifier and missed that shape. This is a static semantic
gate, not a process sandbox.
Adds gate regression tests that fail against the pre-fix code, plus a
clean-script test guarding against over-blocking.
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