Fixes #27158: restore tag_usage prefix-LIKE index on Postgres

The 1.11.0 perf migration (#23054) added four `WHERE state = 1` partial
indexes on tag_usage; #24063 dropped the matching `state = 1` predicate
from getTagsInternalByPrefix (Suggested-state rows are valid for both
classification and glossary derivation), leaving every partial index
inapplicable. Postgres fell back to a parallel seq scan; MySQL was
unaffected because its 1.11.0 indexes were never partial.

Adds a non-partial single-col btree on targetfqnhash_lower (mirrors
MySQL's idx_targetfqnhash_lower) and rebuilds the four partials as
non-partial -- same shape, same INCLUDE columns, predicate coupling
removed so future query changes can't silently invalidate them.

Verified end-to-end against a local Postgres with 50k rows: seq scan
reproduced before the fix (matches reporter's EXPLAIN), bitmap index
scan after, both for inline and prepared-statement paths.

Author: sonika-shah

bootstrap/sql/migrations/native/2.0.1/postgres/schemaChanges.sql

@@ -11,3 +11,30 @@ CREATE TABLE IF NOT EXISTS task_migration_mapping (
 
 CREATE INDEX IF NOT EXISTS idx_task_migration_mapping_new_task_id
     ON task_migration_mapping (new_task_id);
+
+-- Issue #27158: tag_usage seq-scan on Postgres. #24063 dropped the
+-- `state = 1` predicate that 1.11.0's partial indexes required.
+-- Fix: add a single-col index, and drop the `WHERE state = 1` filter
+-- from the existing partials so query changes can't invalidate them.
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_tag_usage_targetfqnhash_lower_pattern
+ON tag_usage (targetfqnhash_lower text_pattern_ops);
+
+DROP INDEX CONCURRENTLY IF EXISTS idx_tag_usage_target_prefix_covering;
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_tag_usage_target_prefix_covering
+ON tag_usage (source, targetfqnhash_lower text_pattern_ops)
+INCLUDE (tagFQN, labelType, state);
+
+DROP INDEX CONCURRENTLY IF EXISTS idx_tag_usage_tagfqn_prefix_covering;
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_tag_usage_tagfqn_prefix_covering
+ON tag_usage (source, tagfqn_lower text_pattern_ops)
+INCLUDE (targetFQNHash, labelType, state);
+
+DROP INDEX CONCURRENTLY IF EXISTS idx_tag_usage_join_source;
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_tag_usage_join_source
+ON tag_usage (tagFQNHash, source)
+INCLUDE (targetFQNHash, tagFQN, labelType, state);
+
+DROP INDEX CONCURRENTLY IF EXISTS gin_tag_usage_targetfqn_trgm;
+CREATE INDEX CONCURRENTLY IF NOT EXISTS gin_tag_usage_targetfqn_trgm
+ON tag_usage USING GIN (targetFQNHash gin_trgm_ops);