Async streaming response wrappers leak an unawaited coroutine when not entered

从 src/browserbase/_response.py:632-637 和 669-685 开始，对比 AsyncResponseContextManager 与同步的 ResponseContextManager。重现 issue 中丢弃 manager 但未进入该 manager 的情况，然后添加一个生命周期测试，证明不会发出未 await 协程的警告，并且进入 manager 仍会执行预期的请求。

Description

The async .with_streaming_response wrappers create the endpoint coroutine immediately, before the returned context manager is entered. If the context manager is conditionally discarded or never entered, Python emits RuntimeWarning: coroutine ... was never awaited.

This differs from the synchronous wrapper, which stores a callable and does not invoke the endpoint until __enter__.

Code reference

• src/browserbase/_response.py:669-685
• AsyncResponseContextManager stores an already-created Awaitable at src/browserbase/_response.py:632-637

Reproduction

import asyncio
import gc
import warnings

from browserbase import AsyncBrowserbase

async def main():
    client = AsyncBrowserbase(api_key="test")
    with warnings.catch_warnings(record=True) as caught:
        warnings.simplefilter("always")
        cm = client.sessions.with_streaming_response.create(project_id="p")
        del cm
        gc.collect()
        await asyncio.sleep(0)
        print([str(w.message) for w in caught])
    await client.close()

asyncio.run(main())

Actual output includes:

coroutine 'AsyncSessionsResource.create' was never awaited

No HTTP request is needed to reproduce this.

Expected behavior

Creating and discarding an unentered response context manager should not allocate an unawaited coroutine or emit a resource warning. The async manager could retain a callable/partial and invoke it lazily from __aenter__, matching ResponseContextManager.

Why it matters

Warnings are promoted to errors in many test suites, and conditional streaming code can produce noisy, nondeterministic resource warnings. This path currently has no direct lifecycle test.