Allow Concat in simple expression contexts

[Copilot]

ConcatFunction was missing IsAllowedInSimpleExpressions => true, causing it to be rejected when EnforceSimpleExpressionConstraint is active — inconsistent with other string functions like Concatenate, Replace, and Substitute.

Changes

• Concat.cs — Added public override bool IsAllowedInSimpleExpressions => true; to ConcatFunction
• BinderTests.cs — Added Concat(["a","b"], Value) to TestExpressionSimpleConstraintValid to cover this case

// Now valid in simple expression contexts:
Concat(["a", "b"], Value, ", ")

[lesaltzm]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jas-valgotar]

✅ No public API change.