Add comprehensive unit tests for RAG package public API

[elbruno]

Problem

The ElBruno.LocalLLMs.Rag package needs comprehensive unit test coverage for all public API surface.

Scope

• RagContext record validation
• RagIndexProgress record validation
• RagOptions defaults and configuration
• RagServiceExtensions DI registration
• InMemoryDocumentStore — add, search, clear operations
• SqliteDocumentStore — add, search, clear, dispose operations
• LocalRagPipeline — index and query workflows

Acceptance Criteria

• Unit tests cover all public types in the RAG package
• Edge cases tested (empty collections, null inputs, boundary values)
• All tests pass on net8.0