chore: improve aho-corasick maintenance path

[vip892766gma]

Summary:

• Add focused unit tests covering edge cases for PatternID/StateID conversions (overflow boundaries, max-value round-trips) and for MatchError/BuildError Display formatting, including a test asserting the precise error message produced when pattern count exceeds the configured limit.
• Keep the change narrow so it is straightforward to review.

Notes:

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