Summary

This PR refactors the LLM provider selection logic, enhances file editing
capabilities with fuzzy matching and improved diff generation, and
introduces provider-specific system prompt extensions.

Changes

• src/services/llm_providers/default.ts:
  • LLM configuration (llmCfg) selection now uses the new PROVIDER
    constant from src/utils/constants.ts instead of directly
    accessing config.provider.
• src/services/tools/fs.ts:

  • The applyFileEdits function has been significantly refactored to
    improve robustness and accuracy of file modifications:

    • Edits are now sorted by their position in the file (in reverse
      order) before application to prevent offset issues.

    • Introduced fuzzy matching logic (findBestMatch,
      calculateSimilarity, applyFuzzyEdit) to locate and apply
      edits when exact matches for old_text are not found. This
      includes normalizing line endings and whitespace for matching.

    • Improved diff output formatting with dynamic backtick counting.

    • Added helper functions: normalizeLineEndings,
      normalizeForMatching, formatDiffOutput,
      sortEditsByPosition, findBestMatch, calculateSimilarity,
      applyFuzzyEdit, and getSurroundingContext for better error
      reporting.

  • The read_multiple_files tool description was slightly
    re-formatted (whitespace).

• src/utils/constants.ts:

  • Added a new PROVIDER constant, which defaults to config.provider
    or "openai".

  • Introduced SYSTEM_PROVIDER_EXTENSION, a record to store
    provider-specific additions to the system prompt (e.g., for
    "gemini").

  • The main SYSTEM prompt now appends the relevant
    SYSTEM_PROVIDER_EXTENSION based on the active PROVIDER.