Add tests for some extensions currently not covered by the regression tests (#11191)

## Problem
Some extensions do not contain tests, which can be easily run on top of
docker-compose or staging.

## Summary of changes
Added the pg_regress based tests for `pg_tiktoken`, `pgx_ulid`, `pg_rag`
Now they will be run on top of docker-compose, but I intend to adopt
them to be run on top staging in the next PRs

docker-compose/ext-src/pg_tiktoken-src/sql/pg_tiktoken.sql

@@ -0,0 +1,24 @@
+-- Load the extension
+CREATE EXTENSION IF NOT EXISTS pg_tiktoken;
+
+-- Test encoding function
+SELECT tiktoken_encode('cl100k_base', 'Hello world!');
+
+-- Test token count function
+SELECT tiktoken_count('cl100k_base', 'Hello world!');
+
+-- Test encoding function with a different model
+SELECT tiktoken_encode('r50k_base', 'PostgreSQL is amazing!');
+
+-- Test token count function with the same model
+SELECT tiktoken_count('r50k_base', 'PostgreSQL is amazing!');
+
+-- Edge cases: Empty string
+SELECT tiktoken_encode('cl100k_base', '');
+SELECT tiktoken_count('cl100k_base', '');
+
+-- Edge cases: Long text
+SELECT tiktoken_count('cl100k_base', repeat('word ', 100));
+
+-- Edge case: Invalid encoding
+SELECT tiktoken_encode('invalid_model', 'Test') AS should_fail;