fix: group small C# nodes to meet minimum block size for indexing

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Summary

This PR fixes an issue where C# code indexing was only capturing using directives and not the actual code content (classes, methods, properties, etc.).

Problem

The code parser was filtering out individual tree-sitter nodes that were smaller than the minimum block size (50 characters). This meant that:

• Individual using directives (e.g., using System;) were too small to be indexed on their own
• When no nodes met the size requirement, the parser would fall back to chunking the entire file content
• This resulted in only using directives being indexed

Solution

Added a _groupSmallNodes method that:

• Groups consecutive nodes of the same type that are individually too small
• Ensures semantically related constructs (like using directives) are indexed together
• Only groups nodes that are close together (within 2 lines)
• Preserves the original behavior for nodes that already meet the size requirement

Testing

• Added comprehensive unit tests to verify the grouping behavior
• Tests cover both successful grouping and cases where nodes shouldn't be grouped
• All existing tests continue to pass

Related Issue

Fixes #6048

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Important

Introduces _groupSmallNodes in parser.ts to group small C# nodes, ensuring they meet the minimum block size for indexing, with tests in parser-csharp-fix.spec.ts.

• Behavior:
  • Adds _groupSmallNodes method in parser.ts to group small nodes, ensuring they meet the minimum block size.
  • Groups consecutive using_directive nodes within 2 lines in C# files.
  • Preserves original behavior for nodes meeting size requirements.
• Testing:
  • Adds parser-csharp-fix.spec.ts with tests for grouping behavior.
  • Tests verify successful grouping and cases where nodes shouldn't be grouped.
  • All existing tests continue to pass.

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