Replace WebApplicationFactory in tests with WebApplicationBuilder

[bkoelman]

All integration tests currently use WebApplicationFactory, which is designed to run tests against an existing app. It sets the content root, solution root and startup project, enables overriding service container entries and more.

But nowadays, most integration tests no longer reference an actual app; they run entirely in a test project. We've been fighting the defaults of WebApplicationFactory to enable that, but keep running into limitations. For example, constraints apply to where the Startup class can be defined. For integration tests that don't reference an actual app, it makes more sense to use the underlying TestServer directly against a freshly built app using WebApplicationBuilder.

This PR replaces WebApplicationFactory usage with the direct approach for all integration tests that don't reference an actual app. It also optimizes database throttling and timeouts to better handle PostgreSQL's maximum connection limits during test runs. The FactoryBridge temporary type was introduced to avoid adapting all existing tests, so this PR focuses only on the test infrastructure.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.96%. Comparing base (dbda7be) to head (c9e6cf0).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2014      +/-   ##
==========================================
- Coverage   92.12%   91.96%   -0.16%     
==========================================
  Files         438      438              
  Lines       15044    15044              
  Branches     2488     2488              
==========================================
- Hits        13859    13835      -24     
- Misses        731      755      +24     
  Partials      454      454              

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