[CHA-0] fix: read rate limit headers case-insensitively; poll in hide/show test

[nijeesh-stream]

TLDR

🧪 Test & lint fails on every PR (e.g. docs-only #241) and on the scheduled master runs. This PR fixes two of the three causes. One is a real SDK bug: with the async client, StreamResponse.rate_limit() returns None. The other is a test race. Risk is low: rate_limit() now finds the headers whatever their case, and headers() is unchanged.

Ticket

• https://linear.app/stream/issue/CHA-5933 (CI is red on the scheduled messages PR [CHA-5933] feat: add scheduled messages #242; no dedicated ticket)

Summary

1. async_chat/test_client.py::test_stream_response: 'NoneType' object has no attribute 'limit'. StreamChatAsync._parse_response passes dict(response.headers), which loses aiohttp's case-insensitive lookup. The server now sends X-Ratelimit-Limit etc., so headers.get("x-ratelimit-limit") misses. The sync client passes the requests case-insensitive dict, so it was unaffected. StreamResponse now looks these headers up case-insensitively. A new offline unit test (test_stream_response_headers.py) fails without the change.
2. test_channel_hide_show (sync and async): assert 0 == 1 on the last assertion. When a message is sent, the backend unhides the channel for other members in a background task (ShowChannelForAllUsersAsync), not inline. The tests now poll query_channels for up to 30s (10s was not always enough for the async test on the shared app).

Not fixed here: test_delete_channels (task did not succeed) fails intermittently on master. The task result shape is unchanged ({"status": "ok"}), so the task either errors or takes longer than 300s on the shared test app. Finding out which needs access to the test app's task results.

Checklist

• black, mypy clean (flake8 only reports a pre-existing B042 in base/exceptions.py, from a newer flake8-bugbear than CI pins)
• The changed code has been covered with unit tests
• The internal documentation (./docs) has been updated (n/a)

🤖 Generated with Claude Code

[nijeesh-stream]

Update after CI: the async test_channel_hide_show still failed on 3.8 and 3.12 with a 30s poll, and test_delete_channels still times out. The sync hide/show test and test_stream_response now pass. Both remaining failures wait on a backend async task (channel unhide, delete channels). stream-chat-go's TestClient_DeleteUsers timed out after 60s at the same time (GetStream/stream-chat-go#405). This points to slow or stuck task processing on the shared CI app's backend shard rather than test timing; someone with access to that app needs to check it. The rate-limit header fix in this PR stands on its own.