Remove "chinese", "japanese", and "korean" from the list of accepted
full-text search analyzer options. Exposing these options commits
ScyllaDB to supporting them long-term — if we ever switch from one
backend search engine to another, CJK analyzers are the most likely
to lose out-of-the-box support, unlike the popular European languages
that are broadly available across text analysis libraries.
Restrict the accepted set now, while FTS is still new, to avoid a
future compatibility burden.
Add a test to check if the CJK language analyzer options are rejected.
Fixes: VECTOR-672
Closesscylladb/scylladb#29877
Move common description logic into a protected helper
`describe_with_target` on `custom_index`, so subclasses can delegate
to it when implementing the `describe()` virtual method.
Introduce `fulltext_index`, a new `custom_index` subclass
for full-text search (FTS).
The index validates that the target column is a text type
(text, varchar, or ascii) and supports two WITH OPTIONS keys:
- 'analyzer': one of standard, english, german, french, spanish,
italian, portuguese, russian, chinese, japanese, korean, simple,
whitespace
- 'positions': boolean controlling whether term positions are stored
`view_should_exist()` returns false — no backing materialized view is
created, matching the CDC-backed pattern used by `vector_index`.
Fixes: SCYLLADB-1517
