Spanner: PostgreSQL Dialect

**Describe the bug**
When running commands against Spanner Database with PostgreSQL dialect, the tool returns an unclear error related to backticks (works fine for Spanner databases with GoogleSQL dialect).

**What version of DVT are you using?**
Ex: 8.5.2

**What type of connections are you using for source and target?**
AlloyDB for source, Spanner with PostgreSQL dialect for target

**Steps to reproduce the behavior**
1.
```
data-validation connections add --connection-name alloydb Postgres \
--host 10.30.0.25 \
--port 5432 \
--user postgres \
--password alloydb123 \
--database test_v2
```

2.
```
data-validation connections add \
--connection-name spanner Spanner \
--project-id reve-ai-26 \
--instance-id nodes-test \
--database-id postgres_2026-03-26_1607-774c
```

3.
```
data-validation validate custom-query row -sc alloydb -tc spanner --source-query "SELECT * FROM public.my_table_01 LIMIT 50" --target-query "SELECT * FROM my_table_01 LIMIT 50" --primary-keys id --hash '*'
```

4. See error
```
Traceback (most recent call last):
File "/home/admin_azulz_altostrat_com/venv/lib/python3.11/site-packages/google/api_core/grpc_helpers.py", line 149, in error_remapped_callable
return _StreamingResponseIterator(
^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/admin_azulz_altostrat_com/venv/lib/python3.11/site-packages/google/api_core/grpc_helpers.py", line 71, in __init__
self._stored_first_result = next(self._wrapped)
^^^^^^^^^^^^^^^^^^^
File "/home/admin_azulz_altostrat_com/venv/lib/python3.11/site-packages/grpc/_channel.py", line 538, in __next__
return self._next()
^^^^^^^^^^^^
File "/home/admin_azulz_altostrat_com/venv/lib/python3.11/site-packages/grpc/_channel.py", line 956, in _next
raise self
grpc._channel._MultiThreadedRendezvous: <_MultiThreadedRendezvous of RPC that terminated with:
status = StatusCode.INVALID_ARGUMENT
details = "[ERROR] syntax error at or near "`""
debug_error_string = "UNKNOWN:Error received from peer ipv4:74.125.135.95:443 {grpc_message:"[ERROR] syntax error at or near \"`\"", grpc_status:3}"
>
```

**Screenshots**

**How are you executing DVT?**
CLI commands on VM

**Additional context**
I believe this is occurring because DVT is configured to generate only GoogleSQL queries, which use backticks, while the PostgreSQL dialect does not support them