chore: improve llm maintenance path

[lphuc2250gma]

Summary:

• Add edge-case unit tests and tighten type annotations for utility helpers in llm/utils.py (e.g., truncate_string boundary cases — empty input, exact-length input, multibyte chars; token-counting helpers with empty/None inputs). Mechanical, contained to two files, no behavior change required.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.