Add GoToSocial ActivityPub integration test scaffold

[Copilot]

• Review GoToSocial OAuth and login flow in upstream code/docs
• Update integration test to use supported authorization_code flow
• Run targeted integration test
• Reply to PR comment with results

Original prompt

Generate a full integration test for the ActivityPub functionality in GoBlog with the fediverse software GoToSocial (GTS - docs.gotosocial.org). The integration test should run a real simulated GTS instance in a Docker container and test the GoBlog ActivityPub functionality that is implemented like following a GoBlog blog or receiving published posts. These tests should ensure that updates to the ActivityPub implementation still fully work with GoToSocial. Later GoBlog might be extended to also allow following fediverse accounts or allow account moving etc.

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[jlelse]

@copilot Take a look at the code of GTS to fix the login and finish the integration test: https://codeberg.org/superseriousbusiness/gotosocial

[jlelse]

@copilot Adjust the code to use the testcontainers framework. Adjust the vscode config files. Adjust the GitHub actions to run the new integration test.

[jlelse]

@copilot Try again