[DPE-7988] Fix check in relation initialisation

[marceloneppel]

Issue

The handler for the database_requested event creates a new user through the primary, but checks if the current unit is has started PostgreSQL. The current unit may be the leader but not the primary, which may lead to errors when trying to access the database.

There is also the case where, for some reason, the K8s PG primary service stops redirecting traffic to the unit for a short period. This makes the charm understand that the cluster has initialised completely and can create users (but it may not due to that weird situation). This still needs further investigation.

Solution

Use the right condition (primary endpoint being ready) in the check.

Port of #1054 for PG 16.

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.68%. Comparing base (31333b5) to head (1d60d9b).
⚠️ Report is 2 commits behind head on 16/edge.

❌ Your project status has failed because the head coverage (68.68%) is below the target coverage (70.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff            @@
##           16/edge    #1061   +/-   ##
========================================
  Coverage    68.68%   68.68%           
========================================
  Files           15       15           
  Lines         4043     4043           
  Branches       582      582           
========================================
  Hits          2777     2777           
  Misses        1058     1058           
  Partials       208      208           

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[taurus-forever]

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