Increase test coverage for diagnostic formatting functions

[davidnbr]

Description

Current test coverage is 38.5%. New functions added in the error formatting feature could use additional test coverage.

Functions Needing More Tests

• sanitizeTerraformANSI() - edge cases with various ANSI sequences
• theme() - caching behavior verification
• renderDiagnosticDetailLine() - various marker patterns and modes
• createGuideReplacer() - replacement accuracy

Suggested Test Cases

sanitizeTerraformANSI

• Nested ANSI codes
• Malformed ANSI sequences
• Mixed color and formatting codes
• Empty strings

Theme Caching

• Verify cache is used on repeated calls
• Verify cache invalidation after toggleRenderingMode()

renderDiagnosticDetailLine

• Dashboard mode with error severity
• Dashboard mode with warning severity
• HighContrast mode with error severity
• HighContrast mode with warning severity
• Lines with underline markers (^, ~)
• Lines with location markers (on file.tf line X)

Underline Pattern

• Various indentation levels
• Mixed ^ and ~ characters
• Edge cases (empty, whitespace only)

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Created from code review of fix_errors_format_default branch