Add async client transport cleanup

[jonathan343]

Overview

This PR adds deterministic resource cleanup to generated clients and HTTP transports. It also preserves the configured transport across operation-level config copies so clients reuse one session and connection pool.

Current State

Each operation deep copies the client config, including its transport. This can create temporary HTTP clients, sessions, and connection pools that are discarded without cleanup. Applications may see repeated Unclosed client session and Unclosed connector warnings during shutdown.

New Pattern

Generated clients and HTTP transports now support close() and async context managers. Operation config copies retain the client-owned transport instead of copying it.

async with AsyncExampleClient(config) as client:
    response = await client.some_operation(input)

Clients can also be closed manually:

client = AsyncExampleClient(config)
try:
    response = await client.some_operation(input)
finally:
    await client.close()

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