[#23922] docdb: Handle colocated tablets correctly in tablet limit ch…

…ecks.

Summary: Currently the tablet limit guardrails treat colocated tables as any other create table request. That is, create table requests to create a colocated table will fail when the universe is close to or at its tablet limit. However, when creating a colocated table besides the first one, the request doesn't actually result in tablets being created and thus the request should succeed. This diff fixes this bug.

Test Plan:
```
./yb_build.sh --with-tests --cxx-test-filter-re tablet_limits_integration_test  --cxx-test tablet_limits_integration_test --gtest_filter 'CreateTableLimitTestRF1.CanAddColocatedTableAtLimit'
./yb_build.sh --with-tests --cxx-test-filter-re tablet_limits_integration_test  --cxx-test tablet_limits_integration_test --gtest_filter 'CreateTableLimitTestRF1.CannotCreateFirstTableInColocatedDatabaseAtLimit'
```

Reviewers: mlillibridge

Reviewed By: mlillibridge

Subscribers: ybase, slingam

Differential Revision: https://phorge.dev.yugabyte.com/D38098