Remove unsafe generic-body convenience properties from OperationSyntax

[Copilot]

OperationSyntax exposed properties (GenericBody, OperandList, SuccessorList, Regions, Attributes, TypeSignatureColonToken, TypeSignatureSyntax, and several derivatives) that implicitly called Body.GetGenericBody(), which throws for custom assembly bodies. This hid the generic/custom body distinction and caused runtime exceptions in otherwise reasonable-looking code.

Changes

• OperationSyntax — Removed all 11 properties that delegated through GetGenericBody(), including the explicit 7 from the issue scope plus audited derivatives: TryGetRawTypeSignature, RawTypeSignature, Operands, Successors. TryGetGenericBody is retained as the safe, explicit accessor.

• Binder, GenericSyntaxBuilder, AssemblySyntaxBuilder — Updated to branch on TryGetGenericBody. Binder falls back to empty collections when no generic projection is available; AssemblySyntaxBuilder.BuildGenericBody throws InvalidOperationException (programming error); GenericSyntaxBuilder skips operations without a generic projection.

• Operation.TypeSignature — Updated to use TryGetGenericBody instead of Syntax.RawTypeSignature.

• Tests — Updated all call sites to explicitly pattern-match on Body:

// Before
var operands = operation.OperandList;

// After — direct cast for known-generic ops
var body = (GenericOperationBodySyntax)operation.Body;
var operands = body.OperandList;

// After — TryGetGenericBody for custom bodies that expose a generic projection
if (operation.TryGetGenericBody(out var body))
{
    var operands = body!.OperandList;
}

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