Fix empty namespaces in C# custom-code references to generated types

[Copilot]

Custom-code references to generated types without customization stubs resolve to Roslyn error symbols, causing model factories to emit invalid names such as global::.ToolConfig.

• Resolution: Fall back to TypeProvidersByName for unresolved, non-generic symbols with empty namespaces. Reuse generated type metadata, preserving nullability, enum semantics, and declaring types.
• Scope: Leave resolved types, qualified symbols, and unmatched names unchanged.
• Coverage: Add regressions for models, fixed/extensible enums, nullable types, nested generic references, and generated factory output.

// Before
IDictionary<string, global::.ToolConfig> toolConfigs = default

// After
IDictionary<string, global::Sample.Models.ToolConfig> toolConfigs = default

• Fixes [Bug]: Custom-code references to generated (un-stubbed) types emit empty namespace ( global::.TypeName ) #11060

[azure-pipelines]

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