[Language] Make LanguageVerificationType an enum class

Convert `LanguageVerificationType` from `enum` to `enum class`,
enforcing type safety and making the code clearer.

Also add unit tests for `NO_PAGE_CONTENT` and
`MODEL_NOT_AVAILABLE` as a follow-up to 5739756.

Bug: 358455699
Change-Id: I30420edac194cc15d7f22a451ab2999cc5a74b8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5780369
Reviewed-by: Basia Zimirska <basiaz@google.com>
Reviewed-by: Trevor Perrier <perrier@chromium.org>
Commit-Queue: Tao Xiong <taox4@illinois.edu>
Cr-Commit-Position: refs/heads/main@{#1340787}