feat(supabase_flutter): default debug logging off under flutter test

[spydon]

What

Makes Supabase.initialize's debug option default to false when running under flutter test, so tests no longer emit INFO/CONFIG log noise unless they explicitly opt in with debug: true.

Why

debug defaults to debug ?? kDebugMode. Under flutter test, kDebugMode is true, so every test that doesn't pass debug: false attaches the log subscription and spams the console (***** Supabase init completed *****, etc.). Rather than sprinkling debug: false across every test, this fixes the default at the source.

This is the more general fix behind #1529 (which added debug: false per call). With this change those manual additions are no longer needed, so #1529 can be closed in favor of this.

How

Reuses the existing web-safe isRunningInFlutterTest getter (already used in supabase_auth.dart, defined via the platform_stub.dart / platform_io.dart conditional import, which reads the FLUTTER_TEST env var):

_instance._debugEnable = debug ?? (kDebugMode && !isRunningInFlutterTest);

Production behavior is unchanged: isRunningInFlutterTest is false outside tests (and the web stub returns false), so the expression reduces to the previous debug ?? kDebugMode.

Verification

• New test/debug_default_test.dart asserts the default is off under flutter test and that explicit debug: true still logs.
• Full packages/supabase_flutter suite: all 59 tests pass with no stray supabase log output; flutter analyze --no-fatal-infos is clean.