fix(postgres): back fulltext indexes with a GIN tsvector index

[HarshMN2345]

Refs appwrite/appwrite#14081

On Postgres, INDEX_FULLTEXT was created as a plain btree on the column, so Query::search could not use it. The search expression called to_tsvector without a config, which is not IMMUTABLE, so it could not be indexed at all. The btree also rejects values above 2704 bytes, which is why saving users with many labels fails (_key_search).

This change uses an explicit config in both search/notSearch and a GIN expression index built from the same helper, so the two expressions always match. A multi-column fulltext index gets one expression per column. Searches only ever use single-attribute fulltext indexes, and a multi-column GIN index can serve a condition on any one of its columns.

Why english rather than simple: the search used to rely on default_text_search_config, which is pg_catalog.english in appwrite/postgres and the official postgres images. Pinning english keeps the same stemming and stop words, so results don't change (for example, room still matches rooms; with simple it matched 0 of 50k rows). Servers set up with a different default would now get English matching. A per-index config could come later.

Migration: fulltext indexes that already exist stay btrees, and from now on search will not use them. They need to be dropped and recreated, ideally with CREATE INDEX CONCURRENTLY on large tables. This PR doesn't include migration tooling.

Verification (appwrite/postgres:0.1.0, PG 18.3, 50k rows; plans captured with auto_explain from Database::find):

• Before: USING btree (note). search('note','zebra') ran as a pkey index scan that filtered every row (24,975 rows removed, 83 ms). Inserting a 3.7 KB value failed with index row size 3712 exceeds btree version 4 maximum 2704.
• After: USING gin (to_tsvector('english'::regconfig, regexp_replace(note, ...))). Same search: Bitmap Index Scan on the fulltext index (0.2 ms). The 3.7 KB insert succeeds. search('note','room') still matches rows containing rooms.
• PostgresTest e2e: 692 tests, the same single pre-existing testCacheFallback failure before and after.

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