Collapse duplicated setup across the suite into six shared helpers under
test/test_helpers/ and rewrite the 28 suites that were open-coding it:
http_fixtures.dart jsonResponse() for http.Response JSON stubs
library_tab_scaffold.dart pumps library tabs under their required ancestors
multi_server_fixtures.dart MultiServerProvider wiring for widget tests
playback_report_fakes.dart PlaybackReportCall + fake report sinks
profile_stack.dart production-shaped profile dependency graph
theme.dart testMonoTokens for fast-settling widget tests
Net -1245 lines with no change in coverage or assertions.
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