Add async context manager support for AIOHTTPClient and HTTPXClient

[jlaportebot]

This PR adds async context manager support (__aenter__ and __aexit__) to both AIOHTTPClient and HTTPXClient, allowing users to use the standard async with pattern to manage the lifecycle of these clients.

Problem

Previously, users had to manually call close_async() to clean up resources, which was error-prone and less ergonomic.

Solution

Added async context manager support to both AIOHTTPClient and HTTPXClient. Now users can use:

async with stripe.AIOHTTPClient() as http_client:
    client = stripe.StripeClient("sk_test_...", http_client=http_client)
    customer = await client.v1.customers.retrieve_async("cus_123")

The context manager automatically calls close_async() when exiting, ensuring proper resource cleanup even if an exception occurs.

Changes

• Added __aenter__ and __aexit__ methods to HTTPXClient
• Added __aenter__ and __aexit__ methods to AIOHTTPClient
• Added comprehensive tests for async context manager support
• Tests verify that context managers work correctly and handle exceptions properly

Testing

Added 4 new tests:

• test_httpx_async_context_manager: Verifies HTTPXClient context manager works
• test_aiohttp_async_context_manager: Verifies AIOHTTPClient context manager works
• test_httpx_async_context_manager_with_exception: Verifies HTTPXClient handles exceptions
• test_aiohttp_async_context_manager_with_exception: Verifies AIOHTTPClient handles exceptions

All tests pass.

Fixes #1796

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