Replace an exact text block in an existing file.

Use this for targeted edits instead of structural patch-style editing whenever possible.

Rules:
- You must read the file first in the current session before editing it.
- `oldString` must match the file exactly, including whitespace.
- If `oldString` appears multiple times, provide more context or set `replaceAll: true`.
- Use `write` for new files and a structural editing tool for multi-file diffs when that capability exists.
