Fix #4332: string[].Contains parse failure under C# 14 + nullable + method init

[jeremydmiller]

Summary

Fixes #4332.

C# 14 with <Nullable>enable</Nullable> wraps captured-closure string[] locals that were initialised from a method-typed return (var v = MakeStrings();) with an extra Convert() before the implicit string[] -> ReadOnlySpan<string> conversion:

s => op_Implicit(Convert(closureField, String[])).Contains(s.Name)

MemoryExtensionsContains.UnwrapConversions only peeled the outer op_Implicit MethodCallExpression, leaving the inner Convert UnaryExpression in place. The receiver then failed TryToParseConstant, parsing fell through to ValueCollectionMember.ParseWhereForContains, and CompileFast'ing the whole Where lambda threw InvalidOperationException: variable 's' ... referenced from scope '', but it is not defined at parse time.

The fix strips Convert/ConvertChecked UnaryExpressions (and op_Explicit) in a loop alongside op_Implicit, so any stacked wrapper combination canonicalises to the captured field and reduces cleanly to a constant. The query then takes the fast IsOneOfFilter path that the literal-init / .ToList() workaround was already hitting.

Test plan

• Added Bug_4332_string_array_contains_under_nullable_method_init with two facts:
  • can_query_when_string_array_local_is_method_initialised — natural-pattern repro using #nullable enable + var values = MakeNames(...); (triggers under net10.0 / C# 14)
  • can_query_when_collection_expression_has_convert_wrapper — builds the failing Convert(closureField, string[]) expression by hand so the bug reproduces deterministically on every TFM
• Both new tests fail on master with the exact InvalidOperationException from the issue and pass with the fix applied.
• Full LinqTests suite green on net10.0: 1260 passed / 1 skipped (pre-existing) / 0 failed.

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