Fix shadowing of type names by variant case record names

[bernardnormier]

Fixes #4896.

The variant enum mapping nests one case record per case inside the union record, so within the union body the case record names shadow same-namespace type names. The case parameter list and the case Encode method now emit fully qualified type names (empty current namespace).

The parameter list regressed in #4877; the Encode method exposure predates it — a tagged (or otherwise lambda-encoded) field whose type name matches a case name produced code that binds the type name to the sibling case record (error CS8121). It went unnoticed because no variant enum had such a field.

The new ShadowedSymbol enum in VariantEnumTests.slice covers both: a self-colliding case (MyStruct(v: MyStruct)), and colliding optional and tagged fields. Compiling the generated code is the regression test; #4898 tracks runtime coverage for struct-typed variant fields, a pre-existing test gap that is independent of shadowing.

The second commit regenerates the checked-in src/ZeroC.Slice.Symbols/Compiler files with the fixed generator: only the version stamps change, confirming the fix restores the exact output these files were generated with.

What's Changed entry

Area: Slice codec

• Fixed the generated code for variant enums whose case names match the type names used by their fields.

[externl]

Looks good.

[InsertCreativityHere]

Looks good to me!

[InsertCreativityHere]

Big fan of adding a comment so we don't accidentally remove this again!
A slight tweak though, since this isn't something that always happens. In reality it should be rare.

Suggested change

	// Inside the union record, the case record names shadow the type names of the enclosing namespace. We set
	// currentNamespace to "" to generate fully qualified type names for the parameter list and the encode method.
	// Inside the union record, the case record names can shadow type names from the enclosing namespace. We set
	// currentNamespace to "" to generate fully qualified type names for the parameter list and the encode method.

[bernardnormier]

Applied, thanks.