Remove external service dependencies in migration tests (#36866)

Fix #36859

Replace live third-party API calls in migration tests with a
fixture-based HTTP mock server. Fixtures are committed so tests run
offline by default; live recording is gated per service on an API-token
env var.

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

services/migrations/_mock_data/TestCodebaseDownloadRepo/GET_%2Fgitea-test%2Ftickets

@@ -0,0 +1,41 @@
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