fix(infra): correct lint_diff relative paths in package makefiles (#36333)

Fixes #36332

Corrected lint_diff/format_diff relative path settings in three package
Makefiles so changed-file linting runs against the correct package
scope. This is an infra-only fix and does not change runtime behavior.

How did you verify your code works?
- make -n -C libs/langchain_v1 lint_diff
- make -n -C libs/text-splitters lint_diff
- make -n -C libs/partners/mistralai lint_diff
- Confirmed only the three Makefiles are changed, with no uv.lock or
pyproject.toml updates.

AI assistance disclaimer: I used AI assistance to draft and review this
contribution.