feat(csharp): do not consider namespace a definition, consider file namespace a scope

What does this MR do and why?

This MR refactors the C# parser to treat file-scoped namespaces as scoping mechanisms instead of definitions. For the following C# code:

namespace MyFileScopedNamespace;

class MyClass {
    void MyMethod() {}
}

The resulting FQN for MyMethod will be MyFileScopedNamespace.MyClass.MyMethod when previously it was MyClass.MyMethod.

The key changes are:

• Namespaces are no longer definitions: The CSharpDefinitionType::Namespace type was removed. Namespaces are now treated purely as scoping constructs and are excluded from the list of definitions.
• File-scoped namespaces define the root FQN: File-scoped namespaces are now correctly identified as the root of the Fully Qualified Name (FQN) for all symbols within the file.

Related Issues

#80 (closed)

Testing

A test was created to ensure namespace resolution.

Performance Analysis

Performance Checklist

• Have you reviewed your memory allocations to ensure you're optimizing correctly? Are you cloning or copying unnecessary data?
• Have you profiled with cargo bench or criterion to measure performance impact?
• Are you using zero-copy operations where possible (e.g., &str instead of String, slice references)?
• Have you considered using Cow<'_, T> for conditional ownership to avoid unnecessary clones?
• Are iterator chains and lazy evaluation being used effectively instead of intermediate collections?
• Are you reusing allocations where possible (e.g., Vec::clear() and reuse vs new allocation)?
• Have you considered using SmallVec or similar for small, stack-allocated collections?
• Are async operations properly structured to avoid blocking the executor?
• Have you reviewed unsafe code blocks for both safety and performance implications?
• Are you using appropriate data structures (e.g., HashMap vs BTreeMap vs IndexMap)?
• Have you considered compile-time optimizations (e.g., const fn, generics instead of trait objects)?
• Are debug assertions (debug_assert!) used instead of runtime checks where appropriate?