fix(compute_ctl): Dollar escaping helper fixes (#11263)

## Problem

In the previous PR #11045, one edge-case wasn't covered, when an ident
contains only one `$`, we were picking `$$` as a 'wrapper'. Yet, when
this `$` is at the beginning or at the end of the ident, then we end up
with `$$$` in a row which breaks the escaping.

## Summary of changes

Start from `x` tag instead of a blank string.

Slack:
https://neondb.slack.com/archives/C08HV951W2W/p1742076675079769?thread_ts=1742004205.461159&cid=C08HV951W2W

test_runner/regress/test_compute_catalog.py

@@ -16,6 +16,9 @@ TEST_ROLE_NAMES = [
     {"name": "role \";with ';injections $$ $x$ $ %I !/\\&#@"},
     {"name": '"role in double quotes"'},
     {"name": "'role in single quotes'"},
+    {"name": "role$"},
+    {"name": "role$$"},
+    {"name": "role$x$"},
 ]
 
 TEST_DB_NAMES = [
@@ -59,6 +62,18 @@ TEST_DB_NAMES = [
         "name": "'db in single quotes'",
         "owner": "'role in single quotes'",
     },
+    {
+        "name": "db name$",
+        "owner": "role$",
+    },
+    {
+        "name": "db name$$",
+        "owner": "role$$",
+    },
+    {
+        "name": "db name$x$",
+        "owner": "role$x$",
+    },
 ]