Adds more sample XSDs for testing (GML, SOAP-WSDL, Loc-ALTO), fixes yet more code generation bugs - #102

Junie's output:
```
### Summary
- Fixed a bug where simple types of `List` variety with enumerated item types (such as `fontStylesType` and `processingCategoryType` in the ALTO schema) were incorrectly identified as enum types, leading to broken validator references (`*Validator.TypeDefinition`), invalid `Enum.Parse` list getters, and non-compiling generated code.

### Changes
- Updated `SOMQueryExtensions.IsEnum` to return `false` for simple types of `XmlSchemaDatatypeVariety.List`. A list variety simple type is a sequence of values (represented in C# as `IList<T>`), not a CLR `enum`.
- Regenerated `GeneratedSchemaLibraries\ALTO\alto-4-4.xsd-g.cs` using the CLI tool with the corrected logic.
- Added a regression test (`T7_ListOfEnumsSimpleType_GeneratesCorrectValidatorAndProperties` in `EnumsCodeGenTest.cs`) verifying that list-of-enums simple types generate clean C# wrapper properties and validator classes without diagnostics.

### Verification
- Built the entire solution (`dotnet build LinqToXsd-TestingSuite.slnf`) cleanly with 0 errors.
- Verified that `ALTO.csproj` compiles cleanly with the regenerated `alto-4-4.xsd-g.cs`.
- Executed unit and regression tests (`fqn:Xml.Schema.Linq.Tests.EnumsCodeGenTest`), which passed successfully.
```

…SD string enums as the item type for an XSD List type. Double checked by junie.

…xsd.

… code being generated.

…igin.md as well.

…arch.Query and Microsoft.Search.Response schemas (which captures most of the XSDs - need to verify with graph however).

…n a graph (standalone, and schemas part of a include/import relationship).

…test in TestUseOfUnknownType

GetDependencies resolves includes using graph.Schema.Single(...) against include.Name, which will throw if the include uses a relative path (e.g. "./foo.xsd") or references a schema that isn't present in the graph. Normalising to Path.GetFileName and using FirstOrDefault (skipping missing targets) makes this resilient to common schemaLocation patterns.

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

… it.

…visited set (it always creates a new returnList), so any include/import cycle will recurse indefinitely (stack overflow). The unused locals (graph, countOfSkips) also suggest the intended visited-set logic was never wired up; this method needs to treat skipList as the accumulator/visited set and short-circuit already-seen schemas.

…ilation test passed: only change is version number.