feat(api)!: migrate to HTTPX2

[apcha-oai]

Makes HTTPX2 the default HTTP client for the next major Python SDK release. See the HTTPX Migration Guide for complete customer-facing migration instructions.

Customer migration

• Default clients: OpenAI() and AsyncOpenAI() use HTTPX2 automatically; API calls, parsed responses, streaming, retries, authentication, and numeric timeouts retain their existing interfaces.
• Dependencies: pip install openai installs HTTPX2 instead of HTTPX. Applications importing httpx through the SDK's former transitive dependency must migrate to httpx2 or install httpx explicitly.
• TLS trust store: HTTPX2 uses the operating-system trust store instead of certifi. This can break certificate verification even with the default client; configure the system trust store, SSL_CERT_FILE, SSL_CERT_DIR, or a custom ssl.SSLContext as needed.
• Custom HTTP integrations: Migrate custom clients, transports, timeout objects, authentication handlers, hooks, request mocks, and instrumentation to their HTTPX2 equivalents. Raw requests, responses, and transport exceptions are now HTTPX2 objects.
• aiohttp: openai[aiohttp] and DefaultAioHttpClient() remain supported through an HTTPX2-native transport without installing HTTPX or httpx-aiohttp.
• Legacy escape hatch: Explicitly installed httpx.Client, httpx.AsyncClient, and httpx-aiohttp clients remain supported when passed through http_client. This compatibility is runtime-only; legacy clients are not supported by static type checkers such as mypy or Pyright. Legacy HTTPX support is provided as a migration aid and may be discontinued.

See httpx2.md for examples and detailed migration cases.

Implementation notes

• Replace the SDK’s default clients and HTTP-facing types with HTTPX2.
• Test legacy HTTPX and aiohttp compatibility separately, including a real request through the legacy aiohttp adapter.

Vendored dependencies
We've vendored a few dependencies in so that we can avoid installing httpx by default for both normal dependencies and dev dependencies. This was the fastest path to unblock migration; we are happy to upstream
these changes if it makes sense for those package's dependencies.

• Fork RESPX under tests/respx2 so existing request-mocking tests work without HTTPX.
  • This is for convenience so we can mechanically rewrite tests.
• Vendor the upstream HTTPX2 aiohttp adapter, including its original license and attribution.
  • This is to avoid aiohttp bringing in httpx by default for now.

Issues

• Consider migrating from httpx to httpx2 #3375