fix: add missing return type annotations

[miachillgood]

Summary

Adds explicit return annotations for Client.capture_exception, the client lifecycle helpers (flush, join, shutdown), and the equivalent module-level helpers. capture_exception now also returns None explicitly if its defensive exception handler is reached.

This is a small incremental typing improvement related to #559.

Validation

• pytest posthog/test/test_client.py::TestClient::test_basic_capture_exception posthog/test/test_client.py::TestClient::test_basic_capture_exception_with_distinct_id posthog/test/test_client.py::TestClient::test_basic_capture_exception_with_correct_host_generation posthog/test/test_client.py::TestClient::test_basic_capture_exception_with_correct_host_generation_for_server_hosts posthog/test/test_client.py::TestClient::test_basic_capture_exception_with_no_exception_given posthog/test/test_client.py::TestClient::test_basic_capture_exception_with_no_exception_happening posthog/test/test_client.py::TestClient::test_capture_exception_logs_when_enabled posthog/test/test_client.py::TestClient::test_flush posthog/test/test_client.py::TestClient::test_shutdown posthog/test/test_module.py::TestModule::test_flush
• pyright typing_smoke.py with # pyright: strict confirmed the updated helper return types as str | None or None.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "Add return annotations for client lifecy..." | Re-trigger Greptile}