fix: remove leading zeros when casting decimal to string

[spena]

Description

What behavior do you want to change, why, how does your patch achieve the changes?

Fixes #4862

Testing done

Describe the testing strategy. Unit and integration tests are expected for any behavior changes.
Added unit tests
Verified manually

ksql> CREATE STREAM rewards(user VARCHAR, reward DECIMAL(100, 0)) WITH(KAFKA_TOPIC='events', VALUE_FORMAT='JSON', PARTITIONS=1, REPLICAS=1);

ksql> insert into rewards(user, reward) values ('u1', 281474976710656);

ksql> select user, CAST(SUM(reward) AS STRING) AS VALUE from rewards GROUP BY user emit changes;
+---------------------------------------------------------------------------------+---------------------------------------------------------------------------------+
|USER                                                                             |VALUE                                                                            |
+---------------------------------------------------------------------------------+---------------------------------------------------------------------------------+
|u1                                                                               |281,474,976,710,656                                                              |

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[agavra]

LGTM!

[agavra]

why do we need this at all? does decimal format not handle integer digits by default?

[spena]

I wasn't sure why this was needed before, but I know now this is used to add the zeros at the left. I removed it now.

[spena]

The test failing is not related to this patch. The previous test passed, and now this one failed. There might a be a problem in master with other code.