[DPE-7502] Improve upgrade tests to support both (in)compatible scenarios

[taurus-forever]

Issue

The newly merged PR (with snap bump) has passed PR testing[1] but failing on release[2].
The reason here is a test imperfection (we've missed the on-Git case):

• on-PR: we are testing incompatible case (due to +dirty charm.py version)
• on-Git: we are testing compatible case, paused after the first unit upgrade

[1] #943
[2] https://github.com/canonical/postgresql-operator/actions/runs/15539934165/job/43748494710

Solution

Improve the test to support both cases: on-PR and on-Git for already merged PRs.

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 70.23%. Comparing base (4ba3220) to head (7fb9350).
Report is 1 commits behind head on 16/edge.

Additional details and impacted files

@@           Coverage Diff            @@
##           16/edge     #955   +/-   ##
========================================
  Coverage    70.23%   70.23%           
========================================
  Files           16       16           
  Lines         3723     3723           
  Branches       541      541           
========================================
  Hits          2615     2615           
  Misses         974      974           
  Partials       134      134           

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[taurus-forever]

Tested as Merged PR in private fork: here and here.

[marceloneppel]

LGTM! Thank you so much, @taurus-forever!