Fix no-pooling session isolation

[hownowstephen]

Summary

• create a fresh requests Session per request when use_connection_pooling=False
• keep pooled clients using the shared cached session
• add ClientBase regression tests for pooled reuse and overlapping no-pooling requests

Context

Applies the concept from #94 to current main. #109 avoided a None.close() traceback, but disabled pooling could still share self._current_session across concurrent requests.

Tests

• .venv312/bin/python -m unittest tests.test_client_base -v
• .venv312/bin/python -m ruff check customerio/client_base.py tests/test_client_base.py
• .venv312/bin/python -m ruff format --check customerio/client_base.py tests/test_client_base.py
• make test PYTHON=.venv312/bin/python

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Note

Medium Risk
Changes HTTP session lifecycle and request dispatching, which can affect performance and retry/connection behavior under load. Adds coverage for concurrent no-pooling requests to reduce regression risk.

Overview
Fixes ClientBase so use_connection_pooling=False creates and closes a fresh requests.Session per request (via a context manager) instead of sharing self._current_session across calls.

Simplifies session management by removing _get_session/_close and related logging, and adds regression tests verifying pooled session reuse and isolation/cleanup for overlapping no-pooling requests.

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[joeybaer]

@cusror review