Replace Extensions with RequestExtensions and ResponseExtensions TypedDicts

[Kludex]

Replaces the loosely-typed Extensions = MutableMapping[str, Any] alias in httpcore2 with explicit RequestExtensions and ResponseExtensions TypedDicts, derived from the extension keys the backends actually read and write.

• RequestExtensions: timeout (a nested Timeout TypedDict with connect/read/write/pool), sni_hostname, trace, target. Declared with extra_items=Any so user/middleware keys still type-check.
• ResponseExtensions: http_version, reason_phrase, network_stream, stream_id, also extra_items=Any.
• extra_items= (PEP 728) lands in stdlib typing only in Python 3.15, so TypedDict is imported from typing_extensions below 3.15 and from typing at/above it; the dependency carries a matching python_version < '3.15' marker.
• Updates _api.py and the sync/async interfaces.py to the new types, and threads RequestExtensions through httpx2.Request.

Note: scripts/check is not fully green yet (pre-existing items unrelated to this typing change).

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[Kludex]

It may make sense to have a single RequestExtensions and ResponseExtensions instead of having 2 of them. And that will probably solve the remaining type checker issues...

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Use typing_extensions.Required for Python 3.10

Because httpcore2 still advertises/supports Python 3.10, these annotations cannot refer to typing.Required: Required was added to stdlib typing in Python 3.11. In Python 3.10 type checking this file reports typing.Required as an unknown member, and runtime consumers that resolve annotations with get_type_hints(..., include_extras=True) will fail to evaluate ResponseExtensions. Import Required from typing_extensions alongside TypedDict for the pre-3.11/3.15 path.

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[cubic-dev-ai]

No issues found across 9 files

_{Re-trigger cubic}