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: adds optional lifecycle helpers for HTTP session cleanup without changing request/response behavior. Main risk is unexpected session closure if callers reuse a client after with/close() without re-sending a request to recreate the session.

Overview
Adds context-manager support to ClientBase via __enter__/__exit__ plus a new close() method that closes and clears the pooled _current_session.

Extends unit tests to verify sessions are automatically closed when using with client: ..., that close() is safe before any request, and that close() resets/cleans up an existing session.

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