fix: isolate context dict per Confidence instance to prevent shared mutable state

[ausbernard]

Description:

Summary

• Moves context from a class-level attribute to an instance-level attribute initialized in __init__, preventing all
  Confidence instances from sharing the same dict
• Changes async_client default from a mutable class-level default argument (httpx.AsyncClient()) to None, instantiating a
  fresh client per instance — fixing the same class of mutable-default-argument bug
• Adds a regression test to assert that context mutations on one instance do not leak to another

Root cause

context: Dict[str, FieldType] = {} was declared at class scope, making it a single shared dict across all instances. Any call to put_context on one Confidence object would silently mutate the context for every other instance in the same process. The same issue existed for async_client, which was evaluated once at class definition time and shared.

Test plan - see issue:

• test_context_is_isolated_per_instance passes — confirms the fix
• Existing test suite passes — no regressions in flag resolution or telemetry behavior

[nicklasl]

Looks good. Let's just make it green 👍

[ausbernard]

👍 I will fix my black formatting real quick and push back up - hold on one sec.

[ausbernard]

✅ done: re-pushing to branch.

confidence-sdk-python on  fix/shared-mutable-context-dict [!] via  v3.11.9 
❯ black --check confidence --exclude="telemetry_pb2.py|_version.py"                                                (base) 
All done! ✨ 🍰 ✨
7 files would be left unchanged.

[nicklasl]

Thanks!