Replace an exact text block in an existing file.

Use this tool for one precise old/new text replacement in one existing file.

Rules:
- Read the file first when possible so `oldString` matches the current content.
- Prefer exact `oldString` text copied from the latest read output.
- If there is no exact match, the tool may apply a unique whole-line match after whitespace normalization (tabs/spaces, leading/trailing whitespace, and repeated internal whitespace). It will not guess through non-whitespace text differences.
- If `oldString` appears multiple times, provide more context or set `replaceAll: true`.
- This tool only modifies existing files.
