Refactor autoIndent.ts: improve code organization and maintainability

[Copilot]

This PR refactors the autoIndent.ts file to make the code cleaner, more concise, and easier to understand while preserving all existing functionality.

Key Improvements

1. Added consistent typing

• Introduced IndentationResult interface to standardize return types across indentation functions
• Improves type safety and code clarity

2. Extracted common validation logic

• validateAutoIndentPrerequisites() - eliminates duplicate validation code (used in 6 places)
• createIndentationResult() - ensures consistent result objects (used in 13 places)

3. Broke down complex functions into focused helpers

Before:

• getInheritIndentForLine(): ~142 lines of complex nested logic
• getIndentActionForType(): ~74 lines with multiple responsibilities

After:

• getInheritIndentForLine(): 47 lines, clean and focused
• getIndentActionForType(): 46 lines, clear logic flow
• 9 specialized helper functions with single responsibilities:
  • getIndentationForFirstNonBlankLine()
  • processIndentationFromPrecedingLine()
  • handleComplexIndentationCase()
  • searchForNonTemporaryIndent()
  • shouldTriggerDecreaseIndentation()
  • getDecreaseIndentation()
  • shouldIndentForAutoClosingPair()

4. Enhanced readability and maintainability

• More descriptive variable names following VS Code guidelines
• Clear JSDoc comments for all helper functions
• Consistent code patterns throughout
• Better separation of concerns

Code Quality Metrics

• Function complexity: Significantly reduced for main functions
• Code reuse: Helper functions eliminate duplication
• Maintainability: Each function has a single, clear responsibility
• Readability: Improved through better organization and documentation

Backward Compatibility

✅ No breaking changes

• All exported function signatures remain unchanged
• All functionality preserved and tested
• Public API unchanged

The refactored code is now much easier to understand, maintain, and extend while following VS Code's coding guidelines.

Fixes #252333.

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