Recheck HTTP/2 flow after acquiring read lock

[lordmauve]

Fixes #1133.

When multiple HTTP/2 uploads exhaust the connection window, a task waiting in _wait_for_outgoing_flow() may queue behind another task on the read lock. That earlier read can deliver the WINDOW_UPDATE the queued task needs, but the queued task currently performs another blocking read without rechecking its window.

This change:

• passes the flow-control stream ID into _receive_events();
• rechecks the stream's effective outgoing window after acquiring the read lock and skips a redundant read when credit is already available;
• applies the change to both the async implementation and its generated sync counterpart;
• adds a deterministic regression test that restores connection credit during read-lock acquisition and verifies no further network read is attempted.

Tests:

• scripts/check
• uv run pytest tests/httpcore2 -q (238 passed)
• async and sync HTTP/2 tests (39 passed)

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing lordmauve:fix/http2-flow-control-race (c5d8da2) with main (4c02c4b)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[cubic-dev-ai]

No issues found across 4 files

_{Re-trigger cubic}