Remove UseProvidedBody string matching from DelegateBodySyntaxExtractor

[Copilot]

DelegateBodySyntaxExtractor searched for lambdas by matching the string "UseProvidedBody" against method names in the syntax tree. This is fragile and unnecessary since BodyGenerationData.RuntimeDelegateBody already tells us whether a delegate body was provided.

Approach

Instead of string-matching a method name, use the data from BodyGenerationData to decide when to extract, and structural position to decide what to extract:

• FluentBodyResult — Added HasDelegateBody flag, set when RuntimeDelegateBody is non-null
• GeneratesMethodGenerationPipeline — Reversed the flow: execute the generator method first, then use HasDelegateBody to conditionally trigger syntax extraction
• DelegateBodySyntaxExtractor — Replaced descendant-node scan for "UseProvidedBody" with structural lookup: find the outermost InvocationExpressionSyntax in the method's return expression and extract its lambda argument

-InvocationExpressionSyntax? invocation = generatorMethodSyntax
-    .DescendantNodes()
-    .OfType<InvocationExpressionSyntax>()
-    .FirstOrDefault(inv =>
-        inv.Expression is MemberAccessExpressionSyntax memberAccess &&
-        memberAccess.Name.Identifier.Text == "UseProvidedBody");
+ExpressionSyntax? returnExpression = GetReturnExpression(generatorMethodSyntax);
+if (returnExpression is not InvocationExpressionSyntax invocation)
+    return null;

Pre-existing fix

Added missing WithCompileTimeConstants and Stage5 implementations in DataMethodBodyBuilders.cs to fix build errors on this branch.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Remove looking for body statement</issue_title>
<issue_description>In a previous PR, #94, a new way to create body was added: to copy the code defined in UseProvidedBody method to the target body.

I like how that works, but I don't like how that is implemented. In DelegateBodySyntaxExtractor, the code looks for memberAccess.Name.Identifier.Text == "UseProvidedBody" which is not very clean.

In BodyGenerationData record, the delegate defined in UseProvidedBody is already stored. Is that not enough to copy the source code? Maybe UseProvidedBody instead of taking Action/Func delegate, should take some "Expression" parameter, or something, so it can capture the exact code, which would remove the need to look for "UseProvidedBody" string?</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Remove looking for body statement #95

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