Convert namespace segments to PascalCase in C# generator

[Copilot]

• Understand the issue and locate relevant code
• Implement PascalCase transformation in GetCleanNameSpace method with minimal refactoring
• Add comprehensive tests for namespace casing
• Build and test the changes
• Verify changes work with integration scenarios
• Address code review feedback to minimize refactoring
• Further minimize changes by preserving original loop structure
• Keep stackalloc buffer allocation - transformation doesn't require variable-length handling
• Remove duplicate IsSpecialSegment logic - delegate to existing method
• Optimize ToString calls - use span for checks, only convert when needed
• Revert global.json to original SDK version
• Fix special segments bug and add test cases for type, array, enum segments

Original prompt

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This section details on the original issue you should resolve

<issue_title>Consider Defaulting to Pascal Case for Generated Namespaces</issue_title>
<issue_description>There is at least one azure spector scenario that is customizing the namespace name: https://github.com/Azure/typespec-azure/blob/main/packages/azure-http-specs/specs/client/namespace/client.tsp. We do not do any casing transformation on the namespace name which is leading to the namespace to be generated as is (all lowercase). In azure, we might consider defaulting to Pascal case though we should also consider if we should have any similar rule for unbranded libraries.</issue_description>

Comments on the Issue (you are @copilot in this section)

@JoshLove-msft This can probably be addressed in TypeFactory.GetCleanNamespace.

• Fixes Consider Defaulting to Pascal Case for Generated Namespaces #8420

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