feat: add lifecycle management support for synchronous and asynchronous clients

[kryachkow]

Applicable issues

#92

Description of changes

Summary

Implements lifecycle management APIs for DIAL clients and pools to enable deterministic shutdown of underlying httpx clients (Issue #92).

What’s included

• Added sync client lifecycle methods:
  • Dial.close()
  • Dial.__enter__() / Dial.__exit__()
• Added async client lifecycle methods:
  • AsyncDial.aclose()
  • AsyncDial.__aenter__() / AsyncDial.__aexit__()
• Added sync pool lifecycle methods:
  • DialClientPool.close()
  • DialClientPool.__enter__() / DialClientPool.__exit__()
• Added async pool lifecycle methods:
  • AsyncDialClientPool.aclose()
  • AsyncDialClientPool.__aenter__() / AsyncDialClientPool.__aexit__()
• Added tests in tests/test_lifecycle.py for all new lifecycle and context-manager paths.
• Updated README.md:
  • Added a Lifecycle Management section with sync/async client and pool examples.
  • Added TOC entry for the new section.
  • Fixed async pool import example to use aidial_client.

Checklist

• Title of the pull request follows Conventional Commits specification

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.