CVE-2026-84382: CWE-409: Improper Handling of Highly Compressed Data (Data Amplification) in pydantic httpx2

### Summary When decoding a compressed response body (`gzip`, `deflate`, `br`, or `zstd`), HTTPX2 fully decompressed each network read before yielding content to the application. A small compressed input could therefore cause a large intermediate memory allocation, even when the application streamed the response to keep memory usage bounded. ### Details HTTPX2's default transport reads the socket in pieces of up to 64 KiB. Before `2.12.0`, each piece was inflated completely into one intermediate allocation before any decompressed bytes were yielded. At DEFLATE's maximum compression ratio of roughly 1032:1, a 64 KiB compressed chunk can expand to about 64 MiB in one allocation. Brotli and Zstandard responses can cause similarly large amplification. Streaming the response did not prevent these transient allocations. ### Impact Applications that fetch resources from untrusted or attacker-influenced servers - such as webhook receivers, link unfurlers, crawlers, SSRF-reachable fetchers, and redirect followers - can experience memory pressure or out-of-memory termination when processing a malicious compressed response. No authentication or user interaction is required beyond issuing a request to the server. ### Mitigation Upgrade to HTTPX2 `2.12.0` or later. Patched versions decompress responses incrementally with bounded intermediate buffers, including responses with multiple content encodings.