Add context manager and close() to ClientBase

[hownowstephen]

Summary

• Adds __enter__, __exit__, and close() to ClientBase
• Both CustomerIO and APIClient inherit this, enabling:

  with CustomerIO(site_id, api_key) as cio:
      cio.identify(id="123", name="Alice")
  # session is automatically closed, no ResourceWarning

• close() is also available standalone for non-context-manager usage

Closes #87

Test plan

• test_context_manager_closes_session — session closed on __exit__, _current_session reset
• test_close_without_session — calling close() before any request is safe
• test_close_resets_session — calling close() after requests cleans up properly
• Full test suite passes (31 tests)
• Lint passes

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Note

Low Risk
Low risk: additive session-cleanup behavior with tests, and it only affects resource lifecycle when close()/context manager is used.

Overview
ClientBase now supports with-statement usage via __enter__/__exit__ and exposes a close() method to explicitly close and clear the internally pooled HTTP session.

Tests are added to verify sessions are closed on context exit, that close() is safe when no session exists, and that _current_session is cleared even if session.close() raises.

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