fix: Unify the request pipeline and retry policy across HTTP clients - #1006

Moves call, the retry loop, and the per-attempt request handling from ImpitHttpClient into HttpClient and HttpClientAsync, leaving Impit as a thin adapter over the transport hooks: send_request, is_retryable_transport_error, is_timeout_error, and close() / aclose() plus context managers. A custom client that overrides call keeps working unchanged and opts out of the shared pipeline.

The same pass fixes:

• Transport errors a retry cannot fix (UnsupportedProtocol, LocalProtocolError, TooManyRedirects) fail on the first attempt instead of burning the whole backoff. Everything else in the impit.HTTPError tree stays retryable, including a bare HTTPError from a body that ends mid-chunk and a ProxyError from a proxy answering CONNECT with a transient status.
• A failure while reading a streamed error response is classified and retried like a send failure, and the response is closed instead of being left open.
• HttpResponse comes from typing_extensions, whose runtime protocol check looks attributes up statically, so isinstance(response, HttpResponse) no longer buffers or rejects an unread streaming body.
• try_import only substitutes failed imports for the optional dependency it names, so an ImportError from the importing module or another dependency is no longer masked, and __all__ omits what a missing extra never provided.

Split out of #1004, which adds the built-in HTTPX client on top of this contract.

✍️ Drafted by Claude Code