## Problem description
During tests we found concurrency issues in the relational table
projections. Tests got flaky if a table (e.g. organization domains) had
too many foreign keys to tables which were projected by another handler.
## Resolution
The projections of the relational tables got merged into one handler to
enforce sequential reduction of events.
## Additional info
- closes https://github.com/zitadel/zitadel/issues/11703
- closes https://github.com/zitadel/zitadel/issues/10810
---------
Co-authored-by: Marco Ardizzone <[email protected]>
# Which Problems Are Solved
This PR provides an example for testing DB reduction to relational
tables without exploiting integration tests.
# How the Problems Are Solved
- Set up a disposable embedded Postgres DB used to store the results of
the calls to the reducers and verify such results
- Add utility method to return transactions from the existing DB pool.
These transactions can be used by the single test cases to insert and
read data from DB
- Add a test to verify that the handler contains the expected reducers
(checking also that they are on the right aggregate type)
- Add tests to verify the single reduce functions. A parent test creates
the required objects to satisfy the foreign key constraints.
# Additional Changes
- Typo fixed in `postgres.DecodeConfig()`
- Discard unneeded `projectionName` parameter for `Executor` function
# Additional Context
- Closes#11430
