Fix #3281: streamable_http (legacy mode): early response.aclose() discards a TCP

[nandanadileep]

Fixes #3281

What changed

Drain the SSE POST response with aread() before aclose() in _handle_sse_response so httpx can return the connection to the pool for reuse.

Verification

python -m pytest -q -x tests/shared/test_streamable_http.py tests/client/test_streamable_http.py tests/interaction/transports/test_streamable_http.py passed

This is a draft so the final diff can be reviewed before it is marked ready.

AI assistance: OpenCode was used to prepare this change; the diff and tests were checked by an automated policy harness.

[github-actions]

This PR has been closed automatically. It's still a draft, but we close those early so you don't put in more time only to have it closed the moment you mark it ready.

This repo only keeps pull requests open when they come from a maintainer, or from a contributor a maintainer has assigned to the linked issue, and you aren't currently assigned to #3281.

If a maintainer assigns you to #3281, this PR reopens on its own and there's nothing more you need to do here. Assignment is a maintainer call based on capacity; comments that only ask to be assigned don't factor in. What does help is engaging on the issue itself by confirming the repro, explaining why it matters for your use case, or describing the approach you'd take.

You're welcome to keep pushing commits here (just avoid force-pushing, since GitHub can't reopen a rewritten branch), but that on its own won't get the PR reviewed or the issue assigned, and realistically most auto-closed PRs stay closed. There's no need to open a new PR either way.

CONTRIBUTING.md has the full reasoning, but in short:

• We're a small team with very little capacity to review community PRs right now.
• Many recent PRs are AI-generated with little human review, and reviewing one carefully still costs a maintainer as much time as it ever did. A well-described issue is usually more useful to us than the code.

Maintainers: reopen, remove missing-issue-link, or add bypass-issue-check to override.