test(supabase): verify shared YAJsonIsolate instance across sub-clients

[grdsdev]

Summary

• Adds tests that explicitly verify SupabaseClient shares a single YAJsonIsolate across PostgrestClient and FunctionsClient (the implementation was already correct)
• Verifies that an injected isolate is properly disposed when SupabaseClient.dispose() is called
• Verifies that creating a SupabaseClient without a custom isolate results in correct shared lifecycle management

Root Cause Analysis

The issue reported that each client was creating its own YAJsonIsolate. Reviewing the current code:

• supabase_client.dart:142 — creates _isolate = isolate ?? (YAJsonIsolate()..initialize())
• supabase_client.dart:306 — passes isolate: _isolate to PostgrestClient
• supabase_client.dart:316 — passes isolate: _isolate to FunctionsClient
• supabase_client.dart:277 — disposes _isolate in dispose()

The implementation was already correct. This PR adds test coverage to lock in that behavior.

Testing

• 2 new tests added in packages/supabase/test/client_test.dart
• All 35 tests pass

Acceptance Criteria

• Single YAJsonIsolate created per SupabaseClient instance (already implemented)
• SupabaseClient.dispose() cleans up the shared isolate (already implemented)
• All existing tests pass
• No public API breakage

Closes: SDK-815

---

🤖 Generated with Claude Code

[Vinzent03]

Actually, I would disagree with the current behavior. If you pass a custom isolate to the supabase client, I don't think it should dispose it. Passing a custom one is a way to make isolate handling between many clients more efficient, which is not possible if it gets disposed by the client. The sublibraries like functions and postgrest only dispose their own isolate. While it is needed there, because the isolate typically comes from the supabase client, I think this design should be applied to the supabase client as well.